I had the same problem.
I signed up in June, tried contact plusnet through email and "Live Chat" which is just an automated bot.
When I finally got through to them on the phone they tried convincing me I somehow clicked on to another offer that didn't include a rewards card but broadband was the same price. This didn't seem unusual or surprising to the guy on the phone which gave me the impression that this happens quite regularly surprisingly enough.
firstname.lastname@example.org are the best people to get answers from, plusnet are awful to try and contact and have no email address to complain to.
Best of luck
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