It sounds like other network services are working on the device? What specifically happens when you click on “Sync” from CommCare’s home screen?
It’s helpful to confirm that the phone works as expected when navigating to https://www.commcarehq.org in the phone’s browser to rule out problems with network security certificates. From CommCare’s home screen you can also click the three dot-menu and navigate to “Advanced -> Connection Test” to potentially get more information.
Another useful thing to check would be that the first unsent form in your queue isn’t corrupted. From CommCare’s home screen you can click the ‘three dot’ menu in the upper-right, and choose “Saved Forms”, then click on “Filter By: All completed forms” and change the filter to “Only Unsent Forms.” The form at the bottom of this list should now be the “next form” that needs to be sent from the queue. If you “long click” that form, you will get the option to “Scan Record Integrity,” which may indicate that there is an issue with the file system which has affected the record. If so you can “Quarantine” that form to allow the rest of the forms to resume sending.