I’m guessing you have Comcast? Maybe Charter? Post the model and we can have a look. If its a dedicated modem, you will probably be fine. The key part is having a dedicated router.
As for connections, here is a picture of the back of the router
The far left is power. Then you have the two USB ports. Then you have a single ethernet port and then 4 ethernet ports. The first one connects to your modem. This is the one bringing in the internet service from the modem. The 4 yellow ethernets distribute the network. Your Tablo would connect to one of them.
A note on the media server. If you are planning for Plex, you will not not be connecting to those USB ports. The Plex media server runs on a computer that is connected to your network, either by wifi or ethernet. For instance, my PMS connects right to the back of that router, in a port next to my Tablo connection.
I actually just explained this to another user
Connecting to those USB ports could be an external storage. This is called a network attached storage (NAS). You probably could work something out with Plex as a front end through this method, but its beyond my experience. The simple way is to have PMS running on a computer, said computer has storage drives with the media, and said computer is on the network (either by wifi or ethernet). You could then install the plex channel on roku and access the server.