Hi! That’s unusual, though I would first advise upgrading the router firmware to the latest version - 8.5.2. This can be done via the Web Admin, through:
System > Firmware.
If the issue still remains, you could check if the cellular module firmware is of the latest version and if not - upgrade it. We had an old forum post on how to do this, though the information is still relevant. If you will be performing an upgrade, make sure that you have an additional WAN connected (physical or Wi-Fi) as all cellular WANs will be disconnected during the upgrade process.
If that still doesn’t help - you can create a ticket and our support team will investigate the cause of the issue.