Why can't ping a wireless broadband modem?

I am in Singapore using M1 and Singtel wireless network. M1 is a partner of Vodaphone, UK, and their wireless network system could be the same.

Singapore is a first world country with the no 1 concentration of millionaires in the world in 2006. So i guess the wireless network system here should not be too much different from the wireless system of UK, USA and Australia.

I had a chat with M1 representative, here is roughly what I learn from him about static IP:
“Wireless network in Singapore cannot have public static IP, it can only have private APN static IP. But we cannot connect with ordinary internet connection to the wireless private APN static IP, we must use another wireless modem with m1 sim card in the same private APN to connect to the wireless private APN static IP, that means two wireless modem is required to connect to each other.”

But in my case i am using dynamic ip, not sure if the above still applies. But I did test no 2 with two wireless modems, still dont work.

The ordinary household USB modem i am testing is designed for plug and play, with very simple GUI with close to nothing settings. So there is no setting for ping blocking or port forwarding.

Thank you.