Does IPv6 work for you? If so the data cap is just namesake

Any traffic over IPv6 is not metered or capped.

If most of your usage is on YouTube or other IPv6 enabled services, some part of your traffic would go over IPv6 by default.
For example, I've used upwards of 300 GB this month till now, but as per BSNL I've used only 170 GB or so. I'm on the Superstar 300 plan.