Doing a search on the relationship of 4G and LTE (two different things) an article showed on, How to Disable LTE on iPhone (and Why You Might Want To).
Quote:
Tap on “Enable LTE” and choose “OFF” (or alternatively, you can set ‘Data Only’ to stabilize some voice calls, or “Voice & Data” as the default)*
I followed the directions above and turned LTE to “OFF” — signal strength improved. I only use the phone for voice and text; cellular data is turned off ~99% of the time. Try it, see if it works.

IPhone 6s is my version of a smarter dumbphone wink Voice, text and a pretty good 12mp camera.