Cant Connect To Speedtouch Modem Wirelessly HELP!

Hey,
Ive changed ISP a few days ago and since i can not connect to my wireless network which was working before i changed ISP.
The ISP sent me a Speedtouch modem, i use my old router but can not connect to the internet on my mac where as i can on my computer(windows).
Does anyone know the soloution to my problem this is really stressing me so i would really appreciate some help.

If you have a new modem, you probably need to setup your router again.
The router is still using the old i.p address from the old modem.

Zomg...so simple thx man =P