Hello!
There are several certification options available when configuring OpenVPN in RUT240. Such as a static key, TLS certificates and password, as well as mixed ones. None of the options have files with the extension .ovpn.
This file is an OpenVPN configuration file, while certificates must be uploaded to the router GUI.
You must contact your service provider to obtain certificates, or consider this file in more detail through text editors; the certificate may already be embedded into it.
I also suggest that you familiarize yourself in detail with the configuration option described on our wiki page:
https://wiki.teltonika-networks.com/view/RUT240_VPN#OpenVPN
https://wiki.teltonika-networks.com/view/OpenVPN_configuration_examples
Best regards, Aliaksandr!