Why does 5G service keep going out on my iPhone?
why does my connection to 5G come and go all the time?
[Re-Titled by Moderator]
iPad, iOS 9
Most likely because your carrier’s network isn’t complete or that extensive.
But also go to Settings/Cellular/Cellular Data Options/Voice & Data. The default is 5G Auto, which means that it will fall back to LTE if the 5G signal is weak.