Macbook Air Wifi problem

Hello,

First of all, I'm sorry if there's already a thread about this, but I googled a long time and could not find an answer. There were similar problems, but never the same. Also, I am fairly good with computers, but not a pro, and I am a PC user with some mac experience. With that said...

My girlfriend bought a macbook air not very long ago and she cannot connect to the internet at home. It works flawlessly however at my place, her dad's place and possibly school (not 100% sure she tried there). At home, the computer connects to the wifi, signal shows full bars, but going on a browser does not work. I tried the diagnostics tool, but that ALWAYS fails at ISP. All lights are green until ISP which is red. All it suggests is to restart devices (modem, router, etc) and that doesn't work. I tried forgetting the network and trying again, but it never works. It's not like dropout problems I've been reading about. Other laptops work, my phone works, so the problem is between the macbook and the modem I would assume. The modem model is arris tm602g, but I couldn't find any info on compatibility with Macs and that modem.

So I'm stumped... any help is appreciated.

Thanks!

EDIT: Forgot to add, if ever it is useful, she does not have airport. Hope I gave all necessary info.

Does it actually get an IP address from the modem/router??

Yeah, it does. When I am on the network options page (before "Assist me" and diagnostics), it says I am connected to the network and it gives me an IP address. It's weird because outside of the diagnstics page, there is no mention of an error (such as an exclamation mark in the menu bar at the top).

Anyone?