The 7 only has 1 antenna band on top and one on the bottom, where as the 6/6s has 2 on top and 2 on the bottom. Stronger signal perhaps with two on the top and two on the bottom ?
Exactly, iPhone 7 has less bands, so poorer signal, its rolling back to 3G as the 4g signal is weak.
Where i use to have a full 4g signal before on my 6s I now have to force my 7 onto 4g and it's only picking up 3 dots of 4g strength so rolling it self back to 3G where I then get full 3G signal strength. (That's how's it designed so it gets maximum battery time)
Apple will stick there's heads in the sand on this one and we're eventually go away!