How does your computer connect to the modem today? It is via a wired Ethernet connection?
The open USB port on the modem won't help you in this case. However, and it sounds like you are thinking along these lines already, you can get a simple router that would also have an Ethernet switch (more than a single port).
Linksys or Netgear are good, reasonably priced options. Expect to spend around $50. You don't necessarily need a wireless router. Usually you get an a wired "DSL/Cable router". That's usually how they are labeled. Be sure it has at least 3-5 LAN ports.
Then you plug an Ethernet cable from the new router to your modem, typically this is labeled WAN or Internet. Plug your computer and Roku into the LAN ports, also with Ethernet cables.
In theory, that should be all that is required and everything else should just work.