Since your modem and router are in one, is the DHCP server hosted by your internet carrier? If that is the case, your carrier will limit your modem to only so many IP addresses.

Also adding to my comment, your internet carrier may also only allow so many MAC addresses registered.

Does either your laptop or PC support custom set MAC addresses? If so, you could change the MAC address on one of those and see if they get assigned an IP address.

It is really driving me mad there should be no reason it's not working, I've tried about a dozen cables (including the one that works for him at home) and tried them on both consoles so it shouldn't be the NIC (unless they both fried). I've powercycled it many times everytime differently, wireless on or off, only the 360 connected, tried it on all ports, static IP outside the DHCP range aswell as in and also while DHCP was off. I just isn't making any sense at all.

MAC is normally disabled but I tried it with enabling it and filling in the correct MAC, didn't work.

Do you mean your MAC address is preset?  The MAC address cannot be "disabled", it is a crucial piece of information in the IP protocol.  Do you know where the DHCP server is?  Is the your modem a modem/switch or a modem/router?

Filling in the correct MAC on what?  I do not know if you can manually set the MAC address of a 360.

I mean MAC filtering disabled, meaning letting everything true, and I meant the fill in the correct 360 MAC address in the gateway settings. (it's a modem and router in one).

(oh and you can change the MAC of the 360 if you wanted)

So if it is a router, the problem will be with the routers settings.  Since the a router is a DHCP host, there shouldn't be anything stopping it from handing out IP addresses except for settings.

EDIT:  How have you confirmed that the 360 isn't getting an IP address from the router?

Not entirely true, my PC's are just working fine, but I can't connect my 360's. It's too bad I can't do a ipconfig all on them.

Edit: you can test the connection on your xbox, everytime it fails while trying to get a valid IP adres (sometimes the router gives me 169.xxx.x.xxx, meaning something is wrong with the leasing, but I should have done everything to overcome that problem.)

If you're getting 169.xxx.x.xxx, that's an automatic private IP. This means your Xbox is not getting anything from the router at all. Is this a router provided by your ISP, or is it one that you own? Have you done anything to change MTU size on the router? Have you tried resetting the router's settings to their factory defaults, then reconfiguring it?

I know your PC's are working fine, but if you can change one of their MAC addresses to something new, you will see if there is a problem with what the DHCP host settings (not all NIC drivers support manually set MAC addresses).

The 169.x.x.x is what is assigned when nothing is received from the DHCP server (default address).

You can also try setting the MAC address of the 360 to the same as your PC, then unplug your PC and plug in the 360 instead.  This is a way of (hopefully) verifying that your 360 ethernet port isn't broken.