When you're subscribing to a VPN service, and they provide a suggested .ovpn file, use that as the basis of your configuration. Place the file in /etc/openvpn on the router, and reference to it in /etc/config/openvpn. Since the .ovpn file contains all the configuration information, /etc/config/openvpn should be only this:
package openvpn
config openvpn 'VPNclient'
option enabled '1'
option config /etc/openvpn/vpn-provider.ovpnwhich of course tells OpenVPN to read the provider's file at startup. You may need to edit that file slightly, using the OpenVPN docs. Also you will need to set up networks and firewall rules to route through the VPN tunnel.