I am not able to replicate your issue.
One thing to try would be to use a different WAN source, either a wired connection or wireless via some access point or a smartphone acting as a hotspot.
For further troubleshooting, I would need a troubleshoot file from your device. First, I would like you to reset the router to factory defaults. Next, configure L2TP client instance, and leave it for a while to establish connection. Then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. Attach the file by editing your question. The logs in the file may provide more details into possible causes of the issue.
Attached files are private and visible only to Teltonika Moderators.
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