Siemens 5890 as gateway?

We've got ISDN at one of our shops (no high speed stuff available, in
Silicon Valley, no less!) and recently had the old Netopia router replaced
with a Siemens 5890. It was replaced by a tech who did nothing more than
plug it in, verify with a laptop that it connected to the 'net, and left.
Great. The original was delivering the 'net to another router, which
distributed (via NAT) the 'net to a number of computers (about 7). But this
rig seems set up to deliver 5 static IP addresses, and I cannot figure out
how to do otherwise.

No problem if I need to send its output to another router, but I tried with
a dlink dir450 (without the 3g card installed) to no avail. Will a plain old
linksys do the job, or do I have to configure the guts of the Siemens 5890?

Thanks-

--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com