Supersonic refuse me to switch to another ISP

I just had it with Supersonic. I have been struggling since April 2021 to cancel my line with Supersonic but with no correspondence from them. Last month via hellopeter I managed to get a reference nr for my cancelation scheduled for 31 August 2021, but this morning the line was still active. My new ISP is supposed to connect me from 1 September 2021 but can't as there is still an active service. I will take all correspondence and evidence that I went out of my way to make contact with Supersonic, following all protocols to make it legit and hand it over to the ombudsman.