So you’ll need to start with the cable modem, the Arris. You’ll need to figure out the IP address that it gives the Leviton router. In the Arris you’ll to create a forwarding rule in which you’ll have to add the IP address of the Leviton router and the port number that Roon uses.
Next step will be to log into your Leviton router and discover which IP address it assigns to your Medialink router. And you’ll have to repeat the process of creating a forwarding rule with the Medialink’s IP address and Roon’s port number.
Then finally you’ll need to create a forwarding rule in the Medialink’s router with the IP address of your Roon Core and the port number that Roon uses.
All these rules need to be set to use the TCP protocol. It’s probably best to turn uPnP off in all routers.
And source IP addresses need to be empty.
I guess Connor is better at helping you, but to me it seems this should set up your connection.
You’ll likely find the forwarding rules options in the advanced settings.