Problem Staying Connected to Service

iPhone 6s iOS 11.1.1, Verizon Unlimited Service

Sometimes my phone disconnects from my service. My icon can show 2, 3 or 4 bars and I can't use apps or send/receive messages, use FaceTime, etc. Then for whatever reason it connects and I receive email, text messages, voice mails, etc. Some of the messages could be an hour or more old, same with voice mail. When it connects I can tell because I will get multiple notifications informing me of emails and messages, etc.

I've tried redoing settings (turning off and on) and shutting phone down and restarting.

Frustrated....