Using Metasploit with 3G USB modem

I have a computer with Linux BackTrack 5 as host and Windows XP as guest on virtual box. I am using Host-Only-Adapter to create virtual LAN and to share USB modem internet as well so that both host and guest are simultaneously connected to internet via single USB modem. Since my WAN IP address dynamically changes, I am using a host-name from no-ip.com to identify my dynamic WAN IP address. With this settings I am able to get meterpreter session using windows/meterpreter/reverse_tcp_dns payload (and port 80) every time I execute payload.exe from Windows XP guest. However, when I execute payload.exe from other Windows XP computer that is on a real LAN, outside my computer, I never get meterpreter sessions. What makes me confused is that the connection is established from netstat on target computer. In this state, if I leave target computer and back to Linux server and scan it with nmap to check port 80 then it said that the port is closed (even though session was not created). Thus: 1) Why weren't the meterpreter sessions created on Linux server while target machine (temporarily) said that connection was established and port 80 on Linux computer is closed as well? (I said temporarily because after several minutes if I check again the netstat on target computer the connection is disappear but port 80 on Linux is still closed.) 2) If server on real-LAN was block the port then why netstat on target computer said that connection was established? if ISP block the port then why I can do it from virtual box? 3) Is it port-forwarding issue? but I doubt since I only use USB dongle and I can do it on virtual box without port-forwarding. 4) Is my 3G USB broadband connection speed not fast enaugh? has anybody success using Metasploit with USB modem?