For any Tandem X2 users who runs into this problem.
Changing the sensor did not fix it. Called Tandem support and got Alex.
The pairing code is not the same as the sensor paring code. Here’s the steps to get the code for pairing pump to app.
Unlock>Options>Device Settings>Bluetooth Settings>Unpair Device> Pair Device (get code, mine was 6 digits) Open app click the bubble to verifiy the pump serial number>enter the pairing code, wait while the circle spins around. If successful the dashboard should come up and after a bit will show connection to the pump.
I did not find anywhere that there was a separate pairing code for the app and for the sensor.
Hopefully the unexpected loss of pairing between pump and app won’t happen to anyone here.
I had this same issue come up a few days ago. I tried everything and finally just deleted the app, rebooted, then reinstalled the app and all was well. No data was lost. Glad you found a solution, @CarlosLuis.