I've run out of ideas...

Have a small business network (192.168.100.0/24) which used to dial
out on a virtually permanent ISDN connection, routed via the server
(198.162.100.100) running Windows NT4 server/ MS Proxy server.

New ADSL line has been added, and is being routed by a USR SureConnect
9003 router/firewall (192.168.100.123).

I want to open up a couple of ports - e.g. 25 and 80 on the external
interface, but re-route the connections to the server.

I have followed all the instructions to re-route requests, and whilst
I can open up port 80 on the router, I can't get any redirection to
work.

Thinking the router might be faulty, I tested a new one on another
network - redirection worked like a dream - but when I tried it on
this network, I failed to get any redirection again. Then tried an SMC
Barricade, which has no firewall. This works fine on network B, but
again, on the first network, I cannot get the redirection to see into
the internal LAN.

I have diabled the windows Firewall, I have tried redirecting to a
client machine with port 25 open, have have disconnected the ISDN line
(still running until the end of the month), and I have now run out of
ideas!

All LAN clients can see the router, the USR router has a ping command,
and it can ping all the internal clients. I've been in touch with the
ISP and they claim not to be port blocking, as evidenced by being able
to map ports other than 80 to the router web server on port 80

I'm somewhat perplexed, as I now have no idea where to even look for a
problem... as a final test before completely tearing my hair out, I'm
going to get an XP Pro machine running a web server, and connect
directly to the router on each of the two networks, with the only
difference being the actual adsl line. If that fails redirection on
one and not the other, what will that actually mean?

Any help gratefully received!

Pete