SIM900A can call and message but can't connect to internet

So i have a sim900a module.
i m using it through software serial on arduino uno for mqtt project.
I tested the module by sending “AT” an it returns “OK”.
i can call receive messages form both sides.
The problem it that it does not connect to the internet.
i was lucky once and had it connected to internet but after that no luck.
i have tried 2 different sim’s of “airtel” both had internet and calling packs.

also after i disconnect a call to the module it shows “NO CARRIER”.
(attached screen shot)

can anyone help ?
thanks !!

Do you have any information on the error its giving?
You should try in terminal first before connecting it to Arduino. First verify if AT commands work when connected to computer by USB-UART converter, then connect to Arduino and run your code.