Most of the ADSL routers out there do not provide a dial backup port.

This is really what you are looking for - a multi port router with a modem interface to dial your ISP

You would have to check the usual suspects to see if any of them provide an ADSL router with a Serial interface as backup - then you could get the router now, use it for dial up now and then change to ADSl once you get it installed.

The ones i would check (in order) YMMV

Linksys
Netgear
D-Link
draytek

etc etc

Craig