fedora core 3 stops network on extern connection from router

used to have an isdn connection from a router talking to a
remote router of the same model (zyxel prestige 100H). Then local router
died and has been replaced by a zyxel 202H. At same time old sco has been
replaced by a fedora core 3.
problem is that fedora stop even pinging the router as soon as the
connection from remote site comes up while the rest of the network happily
talks with it, and to the remote network too. Before the substitution of
the router everything was up and running so its not a routing problem. I
also have to mention that ip address of internal network its not a legal
address (192.2.xxx) but this has never been a problem until now.
traceroute,ping and all else I've tried simply stops when connection is up
and restarts when connection is down