My iPhone 11 will display 1 to 2 bars in an area that I know has full service (from my previous device). Once I toggle Airplane Mode on/off the phone will then display full signal. This happens many times a day, everyday.
I took it in to the Apple Store and they replaced the device. However the issue still remains through updates, resetting network settings, hard reset, and sim change. I constantly have to toggle Airplane mode to get a signal.
A family member has an 11 Pro Max on AT&T and experiences the same issue, albeit my 11 on TMobile is worse. WiFi has no issues.