NEVER go to the store! They work on commission, last time I went to Purchase the iPhone 6 Plus full retail they modified my account to show i signed a 2 year contract. So when the iPhone 7's came out they said i could not update because my contract was not up. a week (and many phone calls and trips later) they finally updated my account to show 2 year contracts don't go with paying full retail for a phone. You see its not in the High School Dropouts that work at the retail store interest to help you unless they get some sort of commission.
I too avoid going to the stores but absolute never is not smart either. Since you tried everything in the other thread the next logical step is to get a different SIM. You can only do this in the store, or call CS and wait a few days to get it delivered. The last 2 times I went to the store was to get a SIM replaced. I make it clear I want nothing else.