Network Issue after flashing Cosmic ROM at Galaxy S Duos
Did you have the required baseband and did you use CWM recovery to install the zip file?
If yes, flash the Stock ROM again and root the phone, intall CWM and install v2 zip file first and then v2.5. 
I have been using Cosmic ROM v2.5 from the past one month and don't have any issues. 
You seem to be having issues only because of some goof up during installation. 
http://forum.xda-developers.com/showthread.php?t=2503230 
He has given the .bin file for the supported baseband. Flash it via odin.