Make sure you are using the latest firmware version. You can download the file here.
Please check the following video on NordVPN configuration on Teltonika routers: https://www.youtube.com/watch?v=jNb1NN_uHZ0
If it still does not work, 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.