Nintendo Switch NAT Traversal Process Has Failed

I was having this same issue with my switch, I’ll give you 3 potential fixes that might help you.

1. Fully shut down your Nintendo switch and restart it. Occasionally this will refresh internet/ connection settings.
2. If shutting down and restarting the switch doesn't work then it's time to reset your router. Resetting your router to factory settings might fix whatever problem may be happening with the server connection and your internet. Just remember that you'll have to set up the TMobile Internet App again and configure your preferred settings (admin password and router password). It's a bit of a hassle but shouldn't take more than 10 minutes to set up and check if the switch works or not.
3. And finally if the other 2 options fail then time to change the DNS setting on the switch. Go to system settings → internet  → internet settings → select your internet network→  change settings→  DNS SETTINGS switch it from automatic to manual. 
   You will have to enter new DNS settings here Primary and Secondary. The first set of DNS is : -> Primary DNS 008.008.008.008             Secondary DNS 008.008.004.004         If this set of DNS doesn't work for you then try this next set: -> Primary DNS 001.001.001.001               Secondary DNS 001.000.000.001         

I sincerely hope this helps you out good luck.