Originally Posted by Freebase Dali

Well we need to figure out whether you're connecting to an actual modem or a router. Ideally, you should have a router connected to the modem, then your computer connected to the router. This will allow more than one computer to utilize the single IP address that gets assigned to the modem from your ISP.
Routers will be able to use NAT, which lets multiple internal computers use private IP addresses to communicate over a single public, routable IP address.

The reason I bring that up is because if it really is a "modem", like a cable modem or what have you, and it doesn't have an integrated router, then only one PC can connect and actually have internet because there's no NAT.

So, that's something you have to confirm.

If it's a router, wireless or otherwise, and you were fine connecting wirelessly but now you can't connect with the wired connection, ask whether the router has MAC address filtering. If it does, this basically requires any device connected to it, wirelessly or not, to have their MAC address entered into the "allow" list. Since your wireless network card and your wired network card are two separate physical entities, and MAC addresses are physically dictated on the device and are unique, you will have a different MAC address for each. And if only your wireless network card's MAC address is allowed, then obviously your wired one won't be.

If this is the case, you will have to get whoever manages the router to add your wired network card's MAC address to the allowed list in the router. You can find this mac address by doing an IPCONFIG /ALL in a command prompt.