Hi,
As I understand, you have a "ovpn" configuration file from Synology device, correct?
If so try following:
First of all, update routers firmware to the latest one without keep settings:
https://wiki.teltonika.lt/view/RUT9xx_Firmware
After update connect to the RUT955 via WinSCP:
File protocol: SCP
Hostname: routers LAN IP, by default 192.168.1.1
User name: root
Password: current routers admin password
After connecting add file VPNConfig.ovpn to /etc/openvpn/
Then connect to RUT955 via SSH and use this command: openvpn --config /etc/openvpn/VPNConfig.ovpn and router should connect to OpenVPN server as a client.
Note.: OpenVPN connection will be up as long as command openvpn --config /etc/openvpn/vpnconfig.ovpn will run if you want to stop OpenVPN on rut simple close SSH connection or use CTRL + C in SSH.
Does this help?