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).
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.