Can't get computer to connect to GSM modem

I'm a student working on a project that involves interacting with a GSM modem via AT commands. I've been having trouble connecting my computer to the modem.

What I know: The modem is on, the drivers are installed, a sim card is in one of the modem's ports, and I see the COM ports in the device manager.

However, when I try to use puTTy to enter with serial, it launches, but it never works. On one of the ports, it lets me type, but it never responds when I use the command "AT". On all the others, it simply won't let me type anything.

I have been stuck on this for a while and any suggestions would be appreciated.