ok after some research i found that i need to forward my port 5900 through my NAT router!! so i logged into my 192.168.1.1 page and disabled the SPI firewall protection, now i can remote access my pc through n900 but only if i am in my home network, if i chose any other network i cant connect.

i also saw the sinle port forwarding option but the table there has like 4 columns
internal port // external port // tcp/udp/both // ip address (192.168.1.__) what should i enter in the blank and should i enter port 5900 in the internal port column or external port column???

i got it