I was having troubles when trying to connect to a TCP sever. I don't know exactly how, but the source of my problems is that the STA IP is set to "0.0.0.0". I tried to rewrite this IP, to a valid one, but without success. If you notice, the command AT+CIFSR keeps returning the IP "0.0.0.0".
The only way to return to a valid IP was sending a RESTORE command, as shown in the terminal below.
I think that it's a bug on firmware. Does anyone have another idea?