Seamless transition from Supersonic to another ISP?

Supersonic is our internet service provider (ISP), running on top of a fibre line supp**** by Vumatel.

On 30 June, I notified Supersonic that I will be discontinuing their service. They require a cancellation notice of a calendar month, so I expected their service to continue until the midnight of 31 July. This was confirmed by their call centre agent on 28 July. However, they terminated their service at midnight of 29 July, while still billing me for the whole of July. They confirmed the cancellation on 4 August, and that they will send me the final invoice between 15 and 25 August. Because I’ve had no internet since 29 July, I’ve been trying to get the new ISP to provide it via the existing Vumatel fibre line that Supersonic was no longer using, but have been unsuccessful, bouncing between Vumatel, who say that they cannot activate the line for the new ISP until Supersonic releases it, and Supersonic, who say that they’ve released it. Today I discovered that Supersonic is indeed holding the line until the final invoice is paid, which they haven’t sent yet. (By the way, this information is not available to their call centre agents, to whom it looks like the line has been released.)

I understand that they wouldn’t want to complete the cancellation process until the account is settled, but the timing of this process is entirely under their control and they should have completed the admin before terminating their service. As it stands, I’m sitting with an unusable fibre line, with no internet for 12 days and counting, in what should have been a seamless transition to another ISP.

Basically, Supersonic’s business model tells their customers: “We’ll ***** you over if you try to leave”, while Vumatel’s business model tells Supersonic: “Go right ahead.”