Hello,
Just to clarify - the "tunX" interface is dynamic, meaning that if there is already "tun0" and "tun1" tunnel, adding a new one will make the new tunnel name "tun2". However, OpenVPN has a built-in option to specify a static interface name. This way it'd be possible to assign a static interface name (for example "tun50") so that the VPN tunnel name always point to the correct interface name.
In order to do this, it's recommended to either login to the router and set the interface name of VPN tunnel in the OpenVPN configuration at Services>VPN>OpenVPN [edit the tunnel], find the "Extra options" field and enter the following command:
dev tun50
Save the configuration when done and in the already provided rules specify the following:
uci set network.vpn1='interface'
uci set network.vpn1.ifname='tun50'
uci set network.vpn1.proto='none'
uci set network.vpn1.metric='100'
uci commit
/etc/init.d/network restart
The same is also possible to do in the ".ovpn" configuration file, simply add a line "dev tun50" (without quotation marks) and save the .ovpn file, then upload it again and save & apply the configuration on the router.
This configuration part will make sure that the specified OpenVPN tunnel will always have a static interface name alongside with it. However, like mentioned before, this is only necessary if you're using multiple OpenVPN tunnels or other services which rely on "tun" interface naming method.
Best regards,
Tomas.