It works with DHCP because the router is doing auto negotiating i.e on the WAN interface its configured as "ip address dhcp".

To input the Gateway address, go into configurature mode and input the gateway address like this..

Router (config)#ip route 0.0.0.0 0.0.0.0 x.x.x.x

(X indicating the gateway IP add.)

use your ctrl + Z to go back to your router prompt..After doing that, make sure you can ping the gateway address from there. First attempt might come back 70 or 80%, but keep trying and it would be 100%.

Save your config and you are all set.

Note: Check your config. and make sure you have no ACL blocking IP traffic from that IP block, no ACL blocking ICMP traffic, and make sure all IP addresses are correct.

Thank you guys!

Now the router is working with a static IP. But does anybody know how to solve this?

===============================================

I configured some NAT port forwarding but none of them worked for me. I have a web server behind the router, but now with the NAT setting all connection through port 80 will go to router remote login.

==========================================

PS: I'm not an IT guy, the SDM is the only way I can handle to config the router.

Thanks