Hi @Tommy_Tyrell,
Most users with the Pace 5268AC Gateway and a second downstream router have found success by following these instructions: Easiest Way to Forward Ports on the Pace Plc 5268AC Router
You’ll need to enter the web administration page for the Pace router and create both a TCP and UDP port forwarding rule that points to the Core static IP and port number listed in Roon → Settings → ARC.
You’ll then need to enter the Amplifi mesh router web administration page and create an identical port forwarding rule there, also pointing to the Core static IP and port number listed in Roon → Settings → ARC.
This is effectively “daisy-chaining” port forwarding through both routers while preventing you from exposing any of your network to the broader internet.
Here are a few threads from users with similar setups, for context:
[AT&T Fiber] Pace 5268AC Gateway, Port Forwarding Fails [Workaround - Manual Configuration]
ARC port forwarding ATT Pace router Orbi Wifi, phone app works
Please let us know if these instructions are helpful.