EE 4G Mobile Broadband - AAISP, Unifi USG and Double NAT - advice needed please

I currently have Voneus fixed wireless broadband giving around 20Mbps DL/UL but have recently had a number of issues with a tree blocking my signal to the repeater and Voneus support has been terrible at resolving or even acknowledging the issue.As a result I’ve been investigating and experimenting with 4G mobile broadband to see what speeds are achievable. I’ve invested in an external aerial and Netgear LB2120 and tried every provider with EE (surprisingly) coming out best with around 25-30Mbps DL / 35 Mbps UL.Now, I want to avoid CGNAT on the EE network so I’m using the L2TP tunnel service to AAISP as mentioned elsewhere on here. Unfortunately my existing router (unifi USG) does not support acting as an l2tp client so I have used an old TPLink Archer C2 I had lying about for this.At the moment, I have the following setup:Netgear LB2120 - in bridge mode connected to EE 4G with CGNAT IP address connected to WAN port onArcher C2 (ip of 192.168.11.1) - in router mode as l2tp client (gets correct external WAN ip from AAISP and displays EE CGNAT IP address in secondary connection details) connected to WAN port onUSG - in router mode with static IP on WAN port of 192.168.11.2. My internal network is on 192.168.5.x.Now I know the above introduces double NAT but without pulling out the whole USG setup (which I’m thinking about because of this!!) I wanted to see if I could get port forwarding to work. I have a server running in my LAN and set up a port forward from the Archer C2 to 192.168.11.2 then on the USG to my server IP.No matter what I do, this does not work. I have seen mention elsewhere (a long pistonheads thread about 4G internet) that some routers do not support port forwarding from the L2TP tunnel. Is this the case here? I have also tried setting 192.168.11.2 (the USG ip) on the Archer C2 as the DMZ as well to no effect.What options do I have here? My next step was going to be to remove the USG from the setup to see if i can connect to the server remotely with just the Archer C2 running the network. Before I do, have I missed something obvious I can try?Many thanks for reading, any suggestions are really welcomed!