Planet fitness taking advantage

Planet Fitness is taking advantage.

In October 2021,I paid a cancellation fee of R1600. In February,I received an email with the subject "sorry to see you leave" in which I was given the option to re-activate my cancelled contract and I considered all ties cut as I did not want to re-activate.

Today,I receive a letter of demand saying I owe R1 837? How is this even lawful? H