Inside the app you have to set Public wifi settings to Outgoing/incoming call use and mVoip setting to Outgoing/incoming call use. This way if you're using mobile data or private WiFi it will automatically retrieve the profile info., and it will also automatically switch to between LTE & WiFi if you get a connection. You can see the phone icon in the notification tab change from "LTE" while on mobile data, to "IP" when using WiFi.
If you have your OS7100 correctly setup with public IP, correct ports open on the router, correct firewall rules and incoming data forwarded to the PBX IP, WeVoip should work on private WiFi networks and through mobile data.