Hello,
First, I would suggest to check this configuration example: https://wiki.teltonika-networks.com/view/L2TP_over_IPsec_RutOS
It would help to have server configuration details, especially key exchange proposal settings.
A troubleshoot file from the RUT360 would be helpful too. To generate the file, first, replicate your configuration, 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.
Best regards,