I am noticing the same thing today on my 7 plus. Went out to run some errands and noticed the phone was on 3G and staying there even though I know there's excellent LTE coverage there. I turned cellular off and on but it didn't help. I then did a hard reboot of my phone and it seemed to remedy it.
Then as I was driving back from the store and the same thing happened. Eventually, (after 10 mins or so) it went to LTE. Again. Stopped at the grocery, stuck the phone in my pocket and when I got back to my car it was on 3G again!!! WTF? All of these areas are LTE and I never had issues on my 6. Starting to think there's a problem with the 7/7 plus.