Hello,
Thank you for reaching out!
Unfortunately, in the logs I cannot see anything, that would signal a disconnection. Could you please navigate to System → Administration → Troubleshoot and generate a Troubleshoot file after reproducing the issue. The file can be attached to the original post. It will only be visible to Teltonika moderators.
I'd also recommend changing the Mobile interface MTU to 1500. This can be done by navigating to Network → Interfaces → General → mob1s1a1 (if SIM1 is in use) → Edit the instance by pressing the pencil icon → Advanced settings → Override MTU: 1500.
Best regards,
DaumantasG