It always tries to use weak cellular service.
The Republic Notification which shows the current condition of your network connection will rule how your phone calls are routed (cell or WiFi)
A good starting point:
- What does the Republic Wireless Notification say
- 1 finger swipe down, provide color and fill/no fill of arc with verbiage
- Run the Diagnostic test from Republic Wireless App / (?) / ‘Run diagnostic Test’ … if any warnings or failures, please post screenshot. You may see something like this