My understanding is that Google Fi is a glorified T-Mobile MVNO at this point (i.e. you're a Fi customer but using T-Mobile towers). It used to give you the closest/best connection of T-Mobile, Sprint, or US Cellular, but Sprint no longer exists, and T-Mobile bought US Cellular.
I've restarted my phone and tried the airplane mode strat but no dice I'm completely kicked out....
Perhaps it's time to try Verizon