Double NAT trouble? ISP-provided modem/router + GT-AX6000 (w/ RMerlin)

[john385]

Hi everyone!

Yesterday I was upgraded from a 1Gbps/100Mbps to 2Gbps/100Mbps cable connection.
During the process they (ISP) switched the provided modem/router from Technicolor CGA4236AON to FRITZ!Box 6660 Cable.

While Technicolor worked flawlessy in bridge mode, allowing my GT-AX6000 to fully work as the main router, I'm having insurmountable issues with the Fritzbox 6660.

Fritz and ASUS are connected in a way that a cable runs from the Fritz's 2.5G port into ASUS's (2.5G) WAN port.

I've been with the ISP's tech support in-depth yesterday and they assure me that I'm not on CGNAT and that they activated the bridge mode on the Fritz.
I think everything's OK on their end since this is what I'm seeing on the Fritz:

WAN is up and running. The speed is OK:

WAN is not (CG)NAT-ed and does provide a "proper" "outside"ISP's WAN address looking from the Fritz:

Unfortunately, my ASUS shows its WAN IP as the NAT-ed LAN the Fritz assigned it:

I tried various options, nothing works.
The ASUS always sees (gets) the NAT-ed IP address as its WAN IP (192.168.xx.xx).

I set the ASUS to be the "exposed host" on the Fritz.
Enabled Independent Port Sharing.


The problem is that since yesterday:

...I can't see my Plex server from outside the network
...my Wireguard VPN setup (on ASUS) no longer works (can't see any LAN resources from outside LAN).
...port forwarding that worked flawlessy not doesn't work at all.

etc. , etc. the usual Double-NAT issues.

What is to be done here?

Please help me out.

 

[Tech9]

You may have to ask your ISP or check their support forums.

Can the FRITZ!Box be used as a modem (bridge mode)? | FRITZ!Box 6660 Cable

en.avm.de

Setting up the FRITZ!Box for use with a cable connection | FRITZ!Box 6660 Cable

You want to set up a FRITZ!Box Cable? ✔ Here you'll find the right step-by-step instructions for your connection.

en.avm.de

Seems like this Gateway doesn't have bridge mode available.

 

[tiddlywink]

Looks like only LAN port #2 is for bridge mode.

Also looks like germans figured out how to enable hidden menus (use google translate to read):

Fritz!Box | Cable (6490, 6590, 6591, 6660) Bridge-Mode freischalten - ubiquiti - Deutsches Fan Forum

ubiquiti-networks-forum.de ist eine Platform für ubiquiti produkte - Deutsches Fan Community

ubiquiti-networks-forum.de

"4. Special features of Fritz!Box 6660

In addition to the 4 LAN ports, the Fritz!Box 6660 also has a 2.5 Gbit port. This cannot be used for the bridge connection. This 2.5 Gbit port is marked with the number 1 in the ports on the back. The 4 LAN ports are numbered 2-5.

In the "Bridge Ports" tab, only ports 2-4 can be used. The WAN port shown does not work, and it is not the 2.5 Gbit port.

However, if you now select port 2 for bridge mode, you have to plug the LAN cable into the LAN port numbered 3 at the back. For port 3, plug it into LAN port 4, etc. The ports on the back are therefore always shifted by one number.

Why is that? Bridge mode is no longer available for normal users and a new connection has been integrated into the Fritz!Box. However, the firmware has not been further adapted. For this reason, AVM has given the 2.5 Gbit connection the number 1. Before the 2.5 Gbit connection, LAN connection 2 was actually LAN connection 1 and this LAN connection has always been blocked for bridge mode because it is used to use the Fritz!Box as a client.

Hence the whole shift on the back.

Update: FritzOS 7.26 works and bridge mode remains. The overview has been revised in the "Bridge Mode" tab. The LAN connections now fit again."

 

[Tech9]

If the 2.5GbE port can't be used for bridge mode though - it makes this 2Gbps plan useless.

 

[M75]

Asus is only getting IP from Box as shown Gateway: 192.168.178.1. My guess is ISP had not done the correct setting or Box confirmed can't be set to bridge mode. Worst case, Box has to be the main router.

Sorry just a silly confirmation, GT-AX6000 has 2x 2.5Gbps. Is the cable connect to the correct port? If it's connected to LAN then AX6000 will gets 2.5Gbps from Box and will also be getting IP address from Box.

 

[john385]

View attachment 61333
Asus is only getting IP from Box as shown Gateway: 192.168.178.1. My guess is ISP had not done the correct setting or Box confirmed can't be set to bridge mode. Worst case, Box has to be the main router.

Sorry just a silly confirmation, GT-AX6000 has 2x 2.5Gbps. Is the cable connect to the correct port? If it's connected to LAN then AX6000 will gets 2.5Gbps from Box and will also be getting IP address from Box.
View attachment 61335

Fritz is connected to the WAN port on ASUS (same one previously used by Technicolor modem).

I think I’m screwed.

I had to go from 1Gpbs to 2Gbps - even though I can’t posssibly use it since I don’t have any 2.5G devices in the house.
They *had* to switch the modem.

Is there a way to configure ASUS in such a way that it works without issues even though there’s a Double NAT going on?

 

[bennor]

Is there a way to configure ASUS in such a way that it works without issues even though there’s a Double NAT going on?

Does the FRITZ!Box 6660 support DMZ? If so, as a troubleshooting step, perhaps putting the Asus router into the FRITZ!Box 6660's DMZ may (or maybe not) work.

PS: No idea if the following would apply in your situation.

Setting up a DMZ with the FRITZ!Box | FRITZ!Box 6660 Cable

A DMZ (demilitarized zone) is a special network that can be accessed from the internet and the local network (LAN). A firewall completely blocks access from the DMZ to the LAN. This concept allows you to make server services (such as email servers) in the DMZ available for access from the...

en.avm.de