Hello and
to the forums.
From what I can see this is a switch and not a router.
You will be fine connecting one Pc to it and as you have found out you got internet access.
However, if you connect another computer to the switch you will find that the switch can not handle requests and route data for both connections to the Internet.
What you need is a router.
Nearly all routers have a built in switch which lets you connect all your devices but then the added thing it has compared to the swtich is it's ability to route data between the modem and your various Internet connected devices without getting confused.