https://github.com/RMerl/asuswrt-merlin/wiki/Policy-based-routing
How-To: OpenVPN – selective Routing and Killswitch on Asus with Merlin WRT
Reply
https://www.hugeserver.com/kb/openvpn-multiple-ports/
https://www.howtogeek.com/64433/how-to-install-and-configure-openvpn-on-your-dd-wrt-router/
To configure iptables look here:
https://www.digitalocean.com/community/tutorials/how-to-set-up-an-openvpn-server-on-ubuntu-16-04
https://www.linode.com/docs/networking/vpn/install-openvpn-access-server-on-linux