Try settings - mobile data - mobile data options - enable 4G - off
Await your iPhone to lose data signal then reconnect probably at 3G level. Then do above again but selecting voice & data. In my experience within 2 minutes if there is a 4G signal it will connect to it without the need to power down and on again.
I was using that method before updating to 10.0.3
Now it takes ages to lose signal when you turn 4g off , when you turn it back on still remain on 3G , so looks like a power cycle to get 4g back