What's the correct way to set up a network with multiple upstream routers?

I have a physical network topology that goes from Modem, to OPNSense Router (subnet 192.168.0.0/24), to L2 Managed Network Switch, to Various LAN clients, including TP-Link Router (subnet 192.168.1.0/24) I'll be referencing port numbers indicated in this topology diagram: The reason I have a secondary router (TP-Link Router) is so that I can have a dedicated VPN client upstream and bypass my service's 5-client limit. This is an intended use of the credentials; the service is ExpressVPN, who advertise directly on their site the ability (and, indeed, how-to guides) for setting up their OpenVPN i

Question by Nico on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.