First, try lowering the MTU on the mobile interface. To do so, navigate to Network → Interfaces → Edit mob1s1a1 → Advanced settings. Here, you can assign a custom MTU value. You could experiment with values like 1460, 1360, or 1260.
If the issue persists, please, try restoring the device to factory defaults in System → Backup. Your current configurations will be lost and you will have to configure the device again. However, this should help with potential configuration and migration issues that may have occurred during firmware updates.