Setting up 2 nics on linux NAS between 2 gateways with different subnets

so many tuts I have read since 3 to 4 days only about 2 nics and routing – but go NOT further! Please could someone help?

Description: 2 Routers RA and RB with both with Gateway to DSL (RA IP: 192.168.178.1, RB IP: 192.168.198.1). In between a Linux NAS with Debian/Ubuntu with 2 NICS (eth0 to RA, 192.168.198.27) (eth1 to RB, 192.168.178.28). The linux NAS has a full debian (except X11), accessible by ssh.

Now if I want by an Ubuntu PC access (192.168.178.21) the internet by 192.168.178.28 (NAS address) in 192.168.178.xxx I can’t get to 192.168.198.1. I have done many ip route ad, route etc. even "echo "1 rt2" >> "/etc/iproute2/rt_tables" etc and those echo 1 > “/proc/sys/net/ipv4/ip_forward”` BUT I CAN’T access 192.168.198.xxx by 192.168.178.21.

Now I want just go to Internet from 192.168.178 over 192.168.178.28 (NAS) to 192.168.198.1. Even I just would like to have some scripts on which I can explicitely choose a route (192.168.178.xxx or the 192.168.198.xxx). And if I wanto add third adapter on NAS (192.168.188.x) ? Please who can help me? Thanks in adcance!!!!

my network conf