The first thing I would recommend is that you update the panel version as yours is on 4.2.1. Older firmware version have this issue where panels can lose cellular connection but it was fixed on firmware 4.4.1.
In order to update to the latest version, it must be updated in increments. Please start with 4.3.0 > 4.4.2 > 4.5.2(latest)
To update the panel, please follow the patch tag instructions under each firmware release in the following link