My wife updated her One Plus 6t and lost connection to the Verizon network (my own non-updated 6t did not have any issues). Factory Reset, Verizon Support, OnePlus support did not have a way to fix the problem.
When testing out swapping SIM Cards with Verizon Support, I tried moving the SIM card to slot 2. Everything magically worked after that. Hopefully this helps someone else out.