I have a Ubiquiti EdgeRouter 4 and Xfinity is my main provider. AT&T is my backup. The router is set to use AT&T only if Xfinity fails. Ports 6000 and 6001 are correctly set in the router. I have also tried 6000 and 6001 with NAT forwarding and IP Passthrough Default Server modes on the ATT Modem.
When I simulate a failure with Xfinity and the system switches to ATT, SC takes a while to connect and states ports not mapped. All other internet activity on the network works.
I have used other dual WAN routers. They are setup so that the port forwarding needs to be setup specifically for each WAN interface. Ideally your router will allow the same points to be forwarded from each WAN interface to your computer running SC. Ubiquiti support maybe able to help in better detail.