GT-AXE16000 dual WAN failover mode

[AnClar]

Just got AT&T Fiber service using their BGW320-505 ONT/Gateway. Also have Spectrum cable internet. Trying to setup Dual WAN in failover mode. I have the BGW320 in passthrough mode, and in single WAN mode, that works fine plugged into 10GB #1 port on the 16000. As soon as I plug in the Spectrum cable modem to the WAN port on the router and set to DUAL WAN failover mode, it completely kills the Internet connection. Lights are normal on the router, but speeds are so slow as to be unusable. Also in dual WAN mode with both services plugged in, I cannot access the router UI via the IP from a wired computer. I can get to the router UI via a wireless connection though. It feels like I'm missing something trivial, but I'm stumped ATM.

 

[Bhoot®]

one of the devices our plugging in has DHCP server enabled and serving the same IP subnet range.

 

[AnClar]

one of the devices our plugging in has DHCP server enabled and serving the same IP subnet range

I'm going to have to restrict the addresses being served by the RG's DHCP server and see. I can't completely disable the DHCP server on the RG or the RG will not pass it's public IP to the 16000.

 

[Bhoot®]

I had a similar issue with Sky modem/wifi boxes, Virgin media (not in modem mode) and a few more.
I always set the core network to 192.168.4x/5x/6/x7.0/24.
Not sure of the rest of your setup, might be easier to change the AXE16k LAN network?
My thought process is to get the WAN links working as needed then set the LAN/DHCP for the main network.