(adsbygoogle = window.adsbygoogle || []).push({}); For whatever reason, I can't join Call of duty online multiplayer servers..
Yet I can connect to PSN network fine.

Any ideas?

Do you get a loading screen saying connecting then a message saying "Cannot connect to online services"?

Yes! exactly this

I got that when I got MW3. I tried all the fixes but none of them worked.

The only actual fix I found was to re-install my old router, which makes me think it's something router based.

That's all the help I can offer you right now, if I found out anything else I'll add it.

What do you mea by re-install your router?

I plugged in my old router.
I got a new router from my ISP over the summer and it wouldn't let me go on MW3, so I got my old router out and powered that up and I could get online.

Like Shadoh said, just restart your router.