this (easter) weekend, I had to fiddle around with an Asus WLAN router, a 56k modem (yes, in Germany we have places without DSL or similar things) and OpenWrt. And what should I say? It works! Thanks to OpenWrt.
First of all, you need some packages which you can install with ipkg on the router: chat, ppp, various usb modules (see openwrt wiki for details) and the usb-serial module.
To enable wifi, please also see the openwrt documentation – I had to use WAP, because WAP2 was not supported by a windows client – and downloading some 10MB big files just to make it work would not be funny over 56k modem ;-)
First step: Set the WAN device from the routers WAN port to the modem device, ppp0
config interface wan
option ifname 'ppp0'
option proto dhcp
Second step: Configure the ppp utils – this will show a config for the provider 1&1
Anlegen der /etc/ppp/pap-secrets.
"1und1/1234-567" * "PASSWORD"
Third step: Add some webscripts, to manage the connect and disconnect via browser
If you have questions… feel free to ask ;-)