Sorry, I'm not going to be supporting VPNs, which lies outside the purview of supporting FPL, as far as I'm concerned.
If you enable a killswitch, that means when your VPN goes down so does your FPL service, and then whether your VPN service always handles UDP port traffic and DNS properly are other factors I'm not interested in troubleshooting. Also, you will likely be increasing jitter and pings by using a third party VPN service.
Helping to support FPL is enough without adding additional points of failure.
If you want help with the OpenVPN features in your router and are using Merlin (which I suggest using), then you can try asking at
https://www.snbforums.com/forums/asuswrt-merlin.42/.