I am trying to connect my smart tv and sky box to the internet via 1 port only how can I do this?

If you already have a router, a switch is the option you want.

If you do want to use another router or an old router as a switch, just turn off its DHCP(assigns ip addresses) and connect the current cable to one if its lan ports. If you use the WAN port and leave DHCP on, systems on either side of the device may not see each other. So a system connected to the main router may have issues seeing the 2 devices on the other side of the second router.