iPhone SE 3G connection won't access internet or send iMessages

[Foolacious]

My new iPhone SE won't access the internet or send iMessages when it's in 3G mode. It says I have no connection to the web and must enable iMessages (which is already enabled). My wife's identical iPhone SE (bought one day after mine) has no such problem. We've checked every setting we can think of and they are identically set up. 4G works fine on both phones (as does WiFi), though for some reason both phones will randomly shift back and forth between 4G and 3G, even when we're stationary, like sitting in a restaurant or on a park bench. We've tried restarting my phone, but no change. Any insights, anyone?

[itsrobert]

1. Have you called Apple or your cell phone provider?

Since you recently purchased it, they should be able to provide support.

[Jake2413]

2. Wife got a new phone

A few months ago and same thing happened. She had to return to the store, the salesperson did not activate the SIM card to her account. Not sure of all the technical info, I did not go with her but it was something with the activation at the store.

[Foolacious]

3. Resolution: New SIM card

Thanks for the responses. I took the phone to my service provider and they ended up doing a factory reset and putting in a new SIM card (which I had brought forward from my previous Android phone and, before that, a dumb-phone).