I had an almost idential problem when I got my first ADSL package. I had a WRT54G and ADSL2MUE.
I too had problems; I plugged the modem into the WAN port of the router and got it *partially* working. I say partially because the connection would only survive about half a minute and then cut-out.
I made a thread on here asking for help, and tolien here (thank you very much
) explained that I would need to set the ADSL2MUE to bridged mode only, and the router to use PPPoE.
To do this, first of all access the modem configurator and select connection type "Bridged Mode Only", and you will probably also need to set it to VC connection type with 0, 38 as the dialing things.
Then you can setup the router, set connection type to PPPoE and type in username and password; it won't need anything else.
Ahhh
Ironic isn't quite the right word but there's some nostalgia amongst other things..
Hope this helps, null
edit - My WRT was a v1.1 and I don't know what the interface is like on the v5s so the process may be different - option names should be similar though.