NAT Problems

I've started to encounter problems with my Switch Lite & Switch OLED when i try to play online. I have NAT type B on both consoles so this shouldn't be happening. Not sure what changed within the system in the past 2 weeks to cause this to happen but it'd be great if I could get some help to fix it permanently. I did talk to a Tmobile chat person on Twitter last week who reset my router to factory settings which did fix it for a bit but it'w now having the same issue as before. I have a Belkin router attached to it but it suffers the same problem as the TMobile modem does. Thanks in advance for any help.