What is your panel’s current firmware version? To potentially resolve this you will need to connect the panel to wifi, then update the firmware. A link to the IQ Panel 4 update thread is below.
IQ Panel 4 firmware updates must be applied incrementally. The versions that can be upgraded from for a specific version are called out in instructions for each version.
There is a known issue with cell reconnectivity on certain versions of firmware which has been resolved. If this does not help, that may indicate that the cellular radio/antenna has failed if you should otherwise have signal available there.