I assume this is true, but just to be sure, is the panel on the ground floor or higher and away from large metal sources? Do you have any general cellular reception issues there?
Assuming there should be a good signal there, first try a power down: Settings > Advanced Settings > Power Down. Leave the system off for 20 minutes or so.
After 20 minutes, boot up, wait a few minutes after boot up, and try a cell test. Any change?
If not it may be necessary to run a Master Reset. This option is found under Settings > Advanced Settings > Dealer Code (default 2222) > Installation > Dealer Settings. This would return the panel to factory defaults.