Redirection with 2 NATs

[SaBuJo]

I have a problem and would like to help.

I have a CentOS 5:10 firewall as a network where the Internet is provided via a modem that is routed and already has NAT, there is no option to disable NAT and the modem receives valid operator and dynamic DHCP IP.

eth0 - 192.168.0.2 with gateway 192.168.0.1 <- Modem routed.
eth1 - 192.168.2.1 - LAN

I need to access the internet on my internal network devices. Following example. I'm using the rules that are not allowing me to reach their desired equipment.

-A FORWARD -i eth0 -o eth1 -p tcp -m tcp --dport 80 -j ACCEPT
-A FORWARD -i eth0 -o eth1 -p tcp -m tcp --dport 91 -j ACCEPT
#
-A PREROUTING -p tcp -m tcp -d xxxx -i eth0 --dport 91 -j DNAT --to-destination 192.168.2.203:80
#
# Network Share
-A POSTROUTING -o eth0 -j MASQUERADE

The xxxx is an DDNS host address.

Thanks for any help.