As developer I made different experiment on how to use this Airtel 2G on 3G and 4G network, fortunately am enable to find this easy and reliable way
Firstly subscribe the 2g 2GB for #200 and 6GB for #500 using the code *482#
After that switch on your Data connection and start browsing after about two minutes of using the 2g network without switching off your data switch up your 3G or 4G network and start enjoying the speedily browsing.
Bear with you that the connection must hang up about 10 minutes of browsing with the 3G/4G network.
Don’t lose hope yet, all you need to do is to switch back to your normal 2G network, on and off your flight mode then repeat the above procedure.
Comment if it works out for you
Note: it might not work on your SIM because it selects SIM