Ok, so apparently successful re-subscription can only happen once your existing plan validity expires. Post that the Net Banking payment option is working without issues. If you have an active plan running, you need to wait it out and then shift to subscription model.

A simple suggestion to QB - If existing annual plan holders cannot re-subscribe until plan validity is over. Please disable the subscribe button and put a message saying you can re-subscribe after this validity expires.

You don't have to confuse users this way, where in subscribe button is there but nothing happens after keying in all the data.