RW phones with GSM SIMs can lose their tower registration, especially when moving from one tower area to another. Also, you can find yourself with lots of bars but emergency service only.
It might be that your phone has lost its tower registration. Rebooting the phone will register the phone with nearby towers. If rebooting the phone down’t work you are probably in an area without adequate coverage.
Yes, this is a nuisance, but the problem is not unique to RW. T-Mobile subscribers experience this too. They recommend frequent reboots… It can happen with any make/model phone.