the modem is free from tmnet wan?
go to tmnet/telekom office change the streamyx plan.
give the modem back to them, maybe get a higher bandwidth package for the same price without a modem or stick to the same package, then from the money saved goto buy a USB type adsl router from computer shops.
sometimes the adsl router which is connected via lan port needs extra configuration on older versions of windows. for windows xp its plug and play, choose DSL and everything done for you just put username and password. ;)