Cashback via BACS

[#1]

I've registered 18 months broadband contract from 01/14/2021 via Topcashback.

Terms & Conditions of Plusnet at Topcashback said 'You must have registered your BACS details with us in order to get cashback from this merchant.'

Can you tell me how to register my BACS detail with Plusnet to get cashback please?

[#2]

Cashback from TopCashback is paid out by TopCashback as they’re a cashback site so it sounds like you’ll need to register your bank details with TopCashback.