I got postpaid SIM/account to start with itself, I had actually visited official Airtel store around June last year before all the 4G advertising had started - for change of my mobile SIM but casually asked status of 4G coverage in my area (because I had 4G 3-4 years ago but had cancelled it due to poor coverage) - then an engineer came for demo with CPE and dongle and since speeds were good I signed up with him then and there, provided my ID proofs etc. Account was activated within a day or two IIRC.
I guess postpaid means you need to use the service every month, I never tried to "pause" the service in between so far. Bill cycle is fixed at certain date of the month and I need to call the CC to change the plans or topup with smartbytes because plan change website is not working (smartbytes is supposed to work but this month I had to call them due to some technical issues going on according to CC)