4G dongle that passes external IP to router?

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TheStork

Occasional Visitor
Does anyone know which Asus compatible 4G/LTE dongles pass the external IP address to the router?

I'm trying to set up a AC87U (with Merlin 380.65_2 firmware) where I'm trying to use ddns and OpenVPN server with a 4G USB dongle as the primary (and only) WAN connection. I bought a ZTE MF823 4G dongle as that was on the Asus compatibility list and have got it connected ok. However, although my 4G SIM card gets a public (dynamic) IP address, the AC87U is assigned a local IP in the 192.168.0.x range, i.e. the dongle appears to have its own DHCP server built in, and therefore using ddns does not work. Obviously it's no good trying to use ddns and incoming VPN connections if the dongle won't let the router know what the external IP address is for updating the ddns settings.

Has anyone found an Asus compatible 4G dongle that simply acts as a modem rather than as a DHCP server and firewall in itself?

Thanks in advance.
 

JDB

Very Senior Member
Are you sure it's not your mobile operator giving you a private IP? How are you sure the SIM is getting a public IP?
I'm a telecoms consultant and work extensively in this area, 99% of the time 3G/4G connections get a private IP and NAT in the providers network to public. As such DDNS won't work unless you have a special contract for a real public IP on your SIM.


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JDB

Very Senior Member
It appears your dongle is doing NAT https://support.aa.net.uk/FireBrick_2700_with_4G_ZTE_MF823#ZTE_MF823_and_No_NAT

And turning it off is not simple!

Suggest getting a new dongle which is a simple modem. I would also double check your SIM address assignment though, generally only business and dedicated 'mobile home broadband' deals will get a public IP direct.


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TheStork

Occasional Visitor
It appears your dongle is doing NAT https://support.aa.net.uk/FireBrick_2700_with_4G_ZTE_MF823#ZTE_MF823_and_No_NAT

And turning it off is not simple!

Suggest getting a new dongle which is a simple modem. I would also double check your SIM address assignment though, generally only business and dedicated 'mobile home broadband' deals will get a public IP direct.


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That's the crux of the question: which 4G dongle does operate as a simple modem without doing NAT?

I bought the ZTE MF823 dongle as I read several users having trouble with at least some of the various Huawei 4G dongles for the same reason of doing NAT. The trouble is it seems difficult to know which dongle doesn't do it prior to buying one and trying it. Hence my question to the community, in case someone has got it working.

And thanks for asking me to double check the SIM contract is for a public IP - it specifically is for that: I pay a bit extra a month to the mobile operator to provide a changing but public IP address (Telia 'OpenGate').
 

JDB

Very Senior Member
On a couple of threads I read it seems 'older' dongles allowed for bridge mode, but no-one mentioned a specific model, and on top of that you then need one that also works with your router.
The main issue is when you google for '4g dongle nat' most of the threads talk about CGNAT which you have already solved with the correct contract!
Maybe your mobile operator can give you some advice on dongles they provide as they must have business customers with this need, with luck one might work with the ASUS.
The other option is something like this could work connected to the routers WAN Ethernet port http://www.dovado.com/en/features/bridge-mode
You'd still need a dongle too but may give a wider scope of compatible dongles.


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Last edited:

TheStork

Occasional Visitor
The other option is something like this could work connected to the routers WAN Ethernet port http://www.dovado.com/en/features/bridge-mode
You'd still need a dongle too but may give a wider scope of compatible dongles.

Thanks JDB. Interesting idea on using a Dovado Tiny as a bridging device. The odd thing is that Dovado's dongle modem compatibility list for bridging includes the ZTE MF823 dongle I have. This suggests that somehow by plugging the dongle into that device the Dovado can get rid of the dongle NAT whereas when connected to an Asus router there is no setting available to turn the NAT off... Strange.

What surprises me is the limited amount of info on 4G setups that work. I hadn't thought I'd be the only one trying to set up a router using DDNS on a 4G connection. I had hoped that someone would know a 4G dongle with no built-in NAT that works in the Asus router and suggest "use dongle x - that works straight out of the box".

Anyone?
 

sfx2000

Part of the Furniture
Are you sure it's not your mobile operator giving you a private IP? How are you sure the SIM is getting a public IP?
I'm a telecoms consultant and work extensively in this area, 99% of the time 3G/4G connections get a private IP and NAT in the providers network to public. As such DDNS won't work unless you have a special contract for a real public IP on your SIM.

Perhaps to share a bit of insight - having been there...

Generally telco's will NAT/CGNAT the network - it's not to be mean spirited, but actually for address and service management as mobiles, being what they are, tend to move around across different cell sites, network boundaries, and even networks themselves (e.g. roaming).

For that reason alone, it's really hard to move a static IP across different carrier gateways, where it's really easy to move private non-routable IP's, as we still have the anchoring home agent to move stuff back and forth...

The other upside of NAT'ing mobiles is that we don't want servers on the 2G/3G/4G/LTE network anyways - see above - it's about privacy and security for all customers/users...

So that being said - the Subscriber Account Profile on the network side (this is driven by the billing platform and rateplan) can have a static IP in the user profile within the HSS/HLR - but this is pretty rare outside of general subscriber accounts.

Check with your provider there... and it does come down to two questions - 1) static ip yes/no, and 2) do you allow servers on the network yes/no
 

TheStork

Occasional Visitor
Check with your provider there... and it does come down to two questions - 1) static ip yes/no, and 2) do you allow servers on the network yes/no

Thanks sfx2000. The 4G providers' usual CGNAT is not the issue here. I have a mobile internet contract that specifically gives me a public IP and allows incoming connections. But the dongle won't pass it through to the router - that's the issue. The issue is NAT by the dongle, not by the 4G provider. Hence my search for a dongle that acts as a modem only rather than as a NAT + dhcp server + firewall...
 

TheStork

Occasional Visitor
The other option is something like this could work connected to the routers WAN Ethernet port http://www.dovado.com/en/features/bridge-mode
You'd still need a dongle too but may give a wider scope of compatible dongles.

And that's indeed what I did in the end: I got a Dovado Tiny, acting in bridge mode with the ZTE 4G dongle, with the Dovado LAN end connected to the WAN port of the Asus router. For some reason the Dovado box can work around the CGNAT while Asus couldn't (despite it being the same 4G dongle), and passes the external IP through from the dongle to the Asus AC87U, thus allowing incoming OpenVPN connections via DDNS. It feels a bit overkill to have to run two router devices for one connection, but at least it works.

Thanks JDB for the tip.
 

JDB

Very Senior Member
Thanks for the feedback. Glad you found a solution in the end. Hopefully someone else will find the info useful too.
As you say, very odd that the same dongle allows WAN pass through via Ethernet but not USB (especially when the USB just presents itself as an Ethernet interface to the router anyway).


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