A switch is the best option. If you want to be able to isolate the camera from the HDHomeRun, then you need to use VLANs. Of course this will require two things..... 1) Your switches need to support VLANs and 2) Your firewall/router needs to support VLANs. Obviously if you have to purchase a switch for that location, then getting one that supports VLANs is easy. However the main switch in your house that the garage switch will be connected to will also need to support VLANs. (Otherwise the VLAN tag will be dropped and the network will not be isolated). It's also unknown if your current firewall/router supports VLANs. If it doesn't, then you would have to change out that hardware or firmware as well.
What networking hardware do you currently use? What router and switch models?