Just to clarify, when you say "hangs", you are referring to the mobile connection, not the entire device, correct?
Either way, without having more information it's hard to say what could be the issue. I will ask you to wait for the issue to replicate, then login to the WebUI, navigate to System → Administration → Troubleshoot and generate a troubleshoot file. This file can be attached to the original post and will only be visible to Teltonika moderators.
Factory reset could also be performed to check if the issue disappears. This can be done by navigating to System → Backup.
Awaiting your response.