What kind of a device will allow me to connect it via cat6 Ethernet to two different routers on different subnets? Is it what all those "WAN" ports on the multi-WAN routers expect already that I'll do?
Yes. The multi-WAN routers can (or should) be able to manage any sort of WAN over Ethernet handoff; bridged PPPoE, DHCP on public address, DHCP on private address, static IPs. One of those fits what your ISP router provides to LAN clients.
Personally, I'd take the time and build an overly complicated custom system (and I have!), but off the shelf should work too. The tricky thing if you build it yourself is determining when the links are up; for my system, I have PPPoE as primary and low quota LTE as secondary, so it's easier, if the PPP link is connected and has responded to ping within the last 30 seconds, I use that, if not I use LTE. In your case, you might need to build some other liveness check.
Don't expect perfect balancing either --- sometimes multiple bulk downloads will end up using the same connection and you won't have any using the other one, but statistically things should work out.
Comments
Yes. The multi-WAN routers can (or should) be able to manage any sort of WAN over Ethernet handoff; bridged PPPoE, DHCP on public address, DHCP on private address, static IPs. One of those fits what your ISP router provides to LAN clients.
Personally, I'd take the time and build an overly complicated custom system (and I have!), but off the shelf should work too. The tricky thing if you build it yourself is determining when the links are up; for my system, I have PPPoE as primary and low quota LTE as secondary, so it's easier, if the PPP link is connected and has responded to ping within the last 30 seconds, I use that, if not I use LTE. In your case, you might need to build some other liveness check.
Don't expect perfect balancing either --- sometimes multiple bulk downloads will end up using the same connection and you won't have any using the other one, but statistically things should work out.