Hello,
Could you update your device with 7.3 firmware, which you can download from here, with Keep settings option set to off (keep in mind, that your current configuration will not be carried over to the new version, so make a backup first, if necessary), and see if the issue persists.
Otherwise, I would like you to attach a troubleshoot file to your question. Please, replicate the issue by rebooting the device. Wait for a couple of minutes for the device to boot and SIM pin requirement to appear. Go to Network -> Mobile > SIM1 / SIM2, click 'Save and apply', wait till data connection establishes. Next, navigate to System -> Administration -> Troubleshoot section and download troubleshoot file from there.
Best regards,