If you’re unable to use the app while connected to mobile data, first check that you have a strong signal. This can be monitored by the signal bars in the top right corner of your phone screen, one bar  being a weak signal and four being a strong signal.
Once you’re certain the issue isn’t caused by a weak connection, please ensure your 3Connect app has access to mobile data.
This can be done by following the steps below:
Within the 3Connect app > Settings > Preferences > Please ensure both Use when available and
Allow VoIP Calls under MOBILE DATA NETWORK are pink.