@slamjam 

If port forwarding works across your ISP router, it should work across the ER605, so long as the WAN IP of the ER605 is also set as the DMZ IP on the ISP router AND you ALSO port forward across the ER605.  Your set up is a 'double NAT' which means everything has 2 NAT hops, both of which need forwarding rules.

  @slamjam 

Many cellular modems don't work correctly if you don't allow them to assign their clients IP via DHCP, just saying.

From what I understand:

(some TIM IP) -->5G Modem--(NAT)..192.168.97.1 --DMZ--> 192.168.97.2..WAN-ER605--(NAT)..192.168.0.1--> Your other devices

For a port forward to work you must configure each port forwarding:

on the 5G Router as such: Port forward External 80 to 192.168.97.2 port 80

then you must also configure on the ER605 WAN a port forward for ER605: WAN port 80 to internal LAN  IP 192.168.0.X port 80 (unless .X is also the DMZ for the ER605)

Hello @slamjam ,

Thank you so much for taking the time to post the issue on TP-Link community!

To better assist you, I've created a support ticket via your registered email address, and escalated it to our support engineer to look into the issue. The ticket ID is TKID230630972, please check your email box and ensure the support email is well received. Thanks!

Once the issue is addressed or resolved, welcome to update this topic thread with your solution to help others who may encounter the same issue as you did.

Many thanks for your great cooperation and patience!