Hello,
Usually denied messages are initiated by the network, rather than the device itself. If your internal modem firmware is older than RG501QEUAAR12A08M4G (e.g. RG501QEUAAR12A07M4G), try performing the following procedure and check if it helps:
Additionally, perhaps the SIM in question was tried on a different device? Have any of the mobile settings (other than the APN) been changed on the device? Perhaps band locking is implemented?
How long have you tried waiting after the connection gets denied to see if it recovers?
As a temporary solution, I would suggest configuring ping reboot by navigating to System → Maintenance → Auto Reboot. Select the action as “modem reboot”, which will only reboot the internal modem instead of the entire device. This will help to minimize the disruption as much as possible. If this action does not help, try changing the auto reboot action to “Reboot”.
Best regards,