How to connect and use GSM Modem

It looks like chat is receiving no reply from the modem/serial port.

Try connecting to the modem using minicom and see what happens when you type stuff in by hand.

You might also want to compile a copy of serlook for your platform.

I will assume that each of the modems have a private IP address and they're NAT through the internet by your provider using a public IP address.

Usually providers block private IP communications with each other (this is called Intra APN communication) the reason is quite obvious: Security. If the subscribers are in the same private network, they likely might be able to access each others IP/ports including Windows shared folders that might not be password protected. Have you tried to ping from one IP to another?

Having said this, your provider should protect you from that scenario that I've just specified.

There are a few workarounds but none are optimal:

1) Buy a private APN from your telco provider. This will allow your SIM cards to communicate freely with each other but not to the world. This is usually a corporate service and for individuals might be a little costly.

2) Connect to an external public IP (VPN/Proxy) responsible to switch the data between the two GSM nodes.

I'm not sure if this is exactly your case but I work for a telco provider and I managed the Packet Switching (PS) Core team for more than 2 years and we dealt a few times with scenarios like those.

I hope this helps.