How to restore serial modem access after installing Ethernet router

[David Jarvie <lists@astrojar.org.uk>]

I access the internet via a serial modem. This worked until I installed an 
Ethernet card connected to an ADSL router. I don't yet have an ADSL account 
with an ISP, so I need my serial modem connection to work. I can still log on 
to my ISP, but I can no longer browse web sites. I used etherconf to 
configure my Ethernet card, setting it up to use DHCP.

How can I make internet browsing work again on my serial modem? There is 
probably some simple bit of configuration needed somewhere, but where ...?

--
David Jarvie.