One thing to try in your case is to increase mobile interface MTU. Login to the your router via WebUI, then navigate to Network > Interfaces. Edit mob1s1a1 interface settings. Switch to Advanced Settings tab. Enter 1500 in Override MTU field. Save & Apply settings
Otherwise, I would like you to attach a troubleshoot file to your question. Please, replicate the issue, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there.