Being specific, if I request lan administrator, to remove the firewall temporarily, and if he would do so, will it solve the problem? Or there are more settings related to router or NAT to be done?
Does any kind firewall exists on router? If yes, how it is configured?
Please do not forget that a good policy established in a firewall works toward the security and benefit of the people behind it, a bad and unmanaged policy just bring a nightmare.
First you have to know that your gateway has several ports to be mapped and the best way to do it, is setting an IP based firewall restriction not a port based one.
This way you can allow Specific IP address use all the ports available on the TCP/IP stack ports like 1720 are for h323 and 1812 and 1813 for accounting and authentications etc.