be sure that you disable NAT on the m0n0wall router. For me it took ages
to find out where :-( You can do this with m0n0wall 1.1
Firewall->NAT->Outbound when you checked "Enable advanced outbound NAT"
and without own mappings below.
The ISDN-router has to know the where the net 192.168.0.0 is. Thus add
"route add -net 192.168.0.0 192.168.2.101". The default route should be
213.x.x.x. For the m0n0wall the default route is 192.168.2.100. The
internal net (192.168.0.0) should have the default route 192.168.0.90,
Hope this helps,