IPhone 7 Plus not connecting to LTE only 3G?

I just got my iPhone 7 Plus. I upgraded from the iPhone 6. My iPhone 6 worked great in my home. But this iPhone 7 doesn't. Same carriers. I've done the service update by dialing ##873283# but it hasn't worked. I also reset my network settings and that did not work. I also reset all my settings and it still isn't working. Everyone else's phones work in my area except mine. I've called customer service took it to the carrier store, called apple, been to apple. No one knows what's going on. But it's only in my area. I called and asked were there service outages or anything in my area. There are none. Has anybody else had this problem with their iPhone 7 Plus? I only had it since June 5th. My iPhone 6 before I switched was working just fine. I'm with boost mobile (Sprint tower).

Settings - cellular - cellular data options - enable LTE - choose voice and data or data.

Or try a hard shut down. That seems to fix most problems.

Did you buy it new from your service provider?
If not, it may be a "gray import" - one intended for a different part of the world.

Different regions use different frequencies for 4G / LTE so it is possible to have 3G but not 4G.

Look at the exact model number. It should be A1661, A1784 or A1785
Those correspond to Americas, Europe and Japan respectively.

Does the phone have Sprint's LTE bands? If it was from another carrier or international device, then it probably doesn't.