Welcome @bnnii
May I ask why OpenVpn is your goto solution to Vpn when NordVpn also supports WireGuard and your router supports this package.
To "shotgun" answer your question however; you can use a cron job (scheduled task) to run a script pinging your OpenVpn connection for connectivity, and the script will restart the IF (interface) if ping replies fall off. That the "Shotgun" answer. The precise script will be tailored to your Interface requirement.
Have a look at the work @pavelgl did for @Surfer2010 with the understanding that we know from your post that 4G LTE is not an alternate WAN interface, and that your request is much simpler. This reading it to give you a primer on what can occur for your solution.
See ya after some due diligent reading of my own.