A few weeks ago my internet connection stopped working, so I contacted my isp and they passed me a new DNS address to put in. I changed it in my PCs network adapter and it worked, so logicaly I changed it on my n900 and at first it worked but not always I always have to connect and disconnect several times before it works. when its not working it gives "address not found error".
It connects to my router but not the internet and it works in school and public places.

Thanks,

Have you tried deleting the connection and letting the N900 get the settings automatically? Besides you shouldn't change the DNS on hosts. The router's job is to share the internet along with the required settings.

I tried deleting the connection and creating a new one without changing the dns but it still didnt work. It was working a few days ago.

try changing it to this

primary 8.8.8.8
secondary 208.67.222.222