OpenVPN Server using Android USB Tether

[clavas]

I am in process of removing my fibre connection as my primary WAN and using USB Android phone tether as my primary (and only) internet connection.

I have successfully set this up and I am achieving faster speeds through my unlimited 4G data connection than through my landline (pings are slightly higher but not an issue)

I have successfully setup VPN clients (going out) but cannot get my OpenVPN server (incoming) to work.
(I suspect I may have the same issue with my NAS server when I come to set this up as this has incoming connections for remote backups and ftp server etc)

The Android USB tethered is a Nexus 6P and my router is an RT-AC86U.

The issue centres around the WAN IP (internet ip) on the tethered mobile and the private WAN IP of the router.

Lets say the Nexus 6P USB modem is 111.111.111.111 (which it isn't!) but my AC68 WAN ip is 192.168.42.161

How do I setup the router/Nexus to allow incoming OpenVPN connections on 1194 via 111.111.111.111 which are accepted by the USB tethered Nexus but handled by the AC86U?
(I have this working successfully when I use the landline fibre connection as my primary WAN and the router has the main internet ip via DDNS)

Any help would be gratefully appreciated.

 

[RMerlin]

Does your phone provide a public IP? Not all of them do. Many providers would also possibly block any inbound connections.

 

[clavas]

Does your phone provide a public IP? Not all of them do. Many providers would also possibly block any inbound connections.

Yes it is with Three UK and it has a public IP stated if I use Whatismyip.com on the phone (or other device connected via the AC86U)

 

[clavas]

Do i need to set something up on the Nexus 6P (or install an app) to forward traffic to the AC86U or does this happen automatically for USB tethered phone?
How do I translate the dynamic WAN IP on the phone (provided by mobile operator) to the private WAN IP which is on the router to get it to accept incoming traffic on certain ports?

Is this possible?