Do have internet connection, but no internet access?

[Terahertz]

I need some help. I'm currently at my mothers house on her laptop because my laptop has some problems with it regarding internet access.

Basically we have a router and a modem (motorola), it used to be that my mother's and my LAN cables would be split by using the router. The router would of course be connected to the modem.

Now, here's my problem: When I split both our LAN cables using the router we're both connected but have no internet access (there would be a yellow triangle with an exclamation mark in the right bottom corner inside of the PC screen thingy).
Now, that's not even -the- problem. When I put my mother's LAN cable from the router directly inside the modem, my mother has internet and can connect to it (which is what I'm writing on now), but if I use the exact same LAN cable, and put it in my laptop, I get the problem that I am connected, but have no internet access.

Basically the 3rd step of the 3 steps that you'd see in the netwerk screen would have a red cross in the middle.

Any help would be appreciated because everything worked fine before my sister tried accessing wi-fi with her phone (which isn't working either).

[Deleted]

Who set it up? It might only give access to specified MAC addresses.

[Terahertz]

How do you mean who set it up? I don't remember that at all, all I know it was working 2 weeks ago and now it's now longer working :S

[2legit2quit]

What are the ethernet adapter settings for the laptop that cannot connect to the Internet?

Start > Control Panel > Set view to "Small Icons" > Network and Sharing Center > Change adapter settings

Right-click the "Local Area Connection" icon, or whichever is connected, and select Properties.

In the white box, click on Internet Protocol Version 4 (TCP/IP). Then click the Properties button.

Is it set to Obtain an IP address automatically or Use the following IP address?

Is it set to Obtain DNS server address automatically or Use the following DNS server addresses?