Hi,

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, 
that's all.

Hope this helps,

Harald