T-Mobile 5G as backup Internet

[SecureCPU]

Hey,
Been using pfsense for some time now. and trying to setup backup internet using T-mobile, but keep having an issue:

WAN1 igb0 - Comcast
WAN2 igb3 - T-Mobile 5G Internet

we're using custom hardware. I've connected T-mobile 5g to pfsense when i first set it up and it worked perfectly.
But not that i'm trying to add it as backup internet its not.

When WAN1 fails, pfsense switches to WAN2, but the internet doesn't work. basically, if someone is on a webpage, that webpage works. but if we go to something else, it doesn't.
The best i got was it seems that something if wrong with the DNS, and while on t-mobile it can't resolve DNS. But we can still ping any external ip address, just not use the domain.

Would anyone have any suggestions?

[tman222]

@SecureCPU - I would check out the DNS section in the multi-wan documentation, and compare against your current configuration:

https://docs.netgate.com/pfsense/en/latest/multiwan/interfaces-and-dns.html

How exactly do you have your DNS configured? Are you using DNS in resolver or forwarding mode?

Hope this helps.