Wow, how complicated. I just setup mine to DHCP, turn on its wifi, unplugged Ethernet and done. Went and gave it a static DHCP lease later for good measure. Running two subnet sound like a pain.
This was the fix. Setting WAN and WWAN to DHCP and assigning a static lease for my desired IP did the trick! Now I don't have to lug my little heater to the router every time I want to change settings or switch pools. Thanks a lot for your help!