iPhone shows No Service when leaving WiFi and WiFi calling

I have a 6S unlocked on T-Mobile with iOS 12.01. When my phone jumps on wifi and connects to T-Mobile WiFi aka WiFi calling, it's unable to find service again on the carrier's (T-Mobile) network when disconnecting from WiFi. Let's say I'm in a building with wifi, and weak network reception. The phone connects to WiFi, jumps on WiFi calling, all is okay. When I leave the building, and the WiFi is no longer available, it simply cannot find a network again. If it's on WiFi calling indoors, and I disable wifi, it won't even attempt to search for a network; it just says No Service. I have to cycle airplane mode to force it. This is a new development with iOS 12.

I reset network settings, and also "restored" my iPhone (although I have restored from a backup as well, and re-wiped network settings, and all settings and preferences), to no avail. I'm dreading having wipe it and not restore mostly because of all the work I would have to put in to restore everything, and I don't even know if this would solve my problem.

I've tried many google searches, and I cannot find anyone describing a similar problem. Has anyone experienced this?