Carrier Settings Update causing iPhone problems?

The "Carrier Settings Update" popup has been alerting me for days but I chose to ignore it only because I wasn't sure what it would do. Today, while in an AT&T store for another reason, it popped up again, so I asked the sales rep if I should do it... He tells me "Yes, that it will help with the way my phone connects to the cell towers & improve my signal strength, etc.", so I did it. Ever since doing so, my phone has been quite frequently saying 'NO SERVICE' or 'NO SIM'! Every time it says so, I will either put it in Airplane Mode or restart my device & that solves the issue for maybe 15-20 minutes or so. I'm running iOS 8.3 on an iPhone 6 Plus 16GB with AT&T. I have already had multiple people tell me their calls have went straight to voicemail. This "Carrier Settings Update" is the only thing I can think of to be the problem, reason being, my phone worked fine before doing so. Is anyone else having this issue?? 😕