My phone won’t keep consistent service?

I have an iPhone 8 64gb with Sprint, had it since May. For months it can’t keep service. It will cycle through in seconds repeatedly between No service, extended, LTE, all while in full coverage LTE areas. It is especially worse when I am traveling. I have done resets and gotten a new sim card but nothing fixes it. Some days it isnt a problem, and others it will do it for hours.