How to enable UDP multicast through NAT

Hello networking gurus,

after some playing and reading stuff on TCP/UDP/IGMP/IPtables with no success - I am asking for an expert advice:

My ISP provides free IPTV service. However as they are FTTB provider they use UDP multicast to distribute the signal.

If I connect my PC to the network (ethernet port with sharp IP connectivity IP by DHCP) I can watch their streams using VLC after opening the UDP port 2314 [on which they stream] in and out and some IGMP stuff that the Windows firewall asks for [which is probably there to allow the configuration packets to reach the last router].

However this does not work when I am behind the WL700gE. Even if I open the UDP ports nothing works, guess that its due to the fact that the router does not get configured (i.e. the ?IGMP? request from the client does not get through WL700ge to the last router) and I am not sure what should I do to
a) enable the IGMP to get through
b) allow the UDP stream to get translated to my local network

I have even tried the VLC client for WL700ge in order to try to receive the stream on WL700ge and "bridge" it to internal network without reencoding which would probably overload the box, but have the same issue. Even if I just try to capture the network stream I get an empty file.

Any idea what am I doing wrong or suggestion what should I try or how should I alter the iptables configuration?

Thanks in advance!