I would like to help you, but I am missing some information.
What type of device you are using (RUT2XX, RUT9XX…)? Firmware version?
OpenVPN should have server and client. Witch side you configured, and having issues?
Are both sides Teltonika devices?
What type of OpenVPN authentication you are using?
More specification on WAN: type (mobile or wired), type of public IP (static, dynamic, shared/private)?