As of right now, it seems the only way to connect a phone with a pc in KDE Connect is via Wi-Fi network. If possible, support for connecting the two devices via mobile data tethering, simply being cable connected, or some sort of alternative would be appreciated.
Thank you for the bug report! This issue has already been filed; please follow the linked bug report for updates on a fix. *** This bug has been marked as a duplicate of bug 421186 ***