Oh man, I can't even begin to imagine how incredibly scary and frustrating that had to be, I am so sorry.
You aren't alone in the signal issues after the iOS update, there is another thread we have been updating over here- Poor LTE Signal after updating to iOS 11
The long and short of it is it seems to be something impacting not just T-Mobile but all carriers, and Apple has yet to formally recognize the issue.
I would suggest contacting care and having a ticket filed, I only say that because I hope the more reports that come in, the faster it will be recognized and hopefully result in a fix. We are doing everything we can our side and passing threads and complaints up to our Apple contacts but are still being met with it isn't a known issue.
We are going to keep that thread I linked you to updated as we get more info, so if you can we suggest bookmaking or following it.
Again, we are so sorry about this but we are working to try and figure out what's going on.