solution:
I have a speedtouch 546i and a wl500gp (in wds mode) and several wrt54gl and belkin behind the wl.
My speedtouch is not in modem mode, just router mode.
But it should not make any difference, because the speedtouch is connecting to INET with user, password anyway.
Hence, you have to set the Asus in Menu: IP Config - WAN & LAN just to Automatic IP, Apply & Finish - thats it.
BTW: if you want to use your asus as webserver aso., please do not forget, that the DMZ settings in the speedtouch menu is nearly hidden.
Newbiefan