Have to re-activate after lost service

i got a replacement six plus on Monday. I work third shift, in a steel building and sometimes lose signal. W the old phone as soon as I would get to where the signal would return so would my service. W the replacmeant six plus now when I lose service I have to completely re-activate the phone. Not just power off and on, I mean input iTunes username and password. Went to local store today and got a new SIM card, after being promised the moon and stars that it would take care of it..... It didn't. So whats the next step??!!

Please advise help growing more and more frustrated w this problem.

TIA

MIKE