EDIT 11/17/2023: First try updating firmware to 2.8.1 per the steps linked below:
Hmm, looking at the account history, I see a manual phone test, and pings to the system requesting a cell response are successful, except when sending them via cell.
In other words, I can send a ping via broadband and request a cell response, we get an acknowledged response.
Try disabling wifi. Then power down for 20 minutes. (Settings > Advanced Settings > Power Down.)
After 20 minutes, boot up the panel, do not connect wifi again yet, just run a cell test a few times a few minutes after boot up. Any difference in response?
After those steps go ahead and enable wifi again.