If the reception is too bad,the phone use edge instead of LTE,I just cannot understand why you need to reboot.
Make sure you have the latest iOS version and the latest carrier update for your wireless provider.
You would likely have the same result by toggling Airplane mode, instead of rebooting.
Not sure if it's the same issue or not, but lately my AT&T iPad has been doing the same thing. A lot of times I wake up in the morning and it will say No Service, I have to reboot to get service. Toggling airplane mode or cellular data on/off does not resolve the issue. Only a reboot.
I was using the latest beta version of iOS, and then restored to the last stable version and the issue continues.