This request goes beyond what most resellers can reasonably provide, since you have no actual standing or authority to act on a customer's behalf with an entirely separate, unrelated company.
Politely explaining this limitation while still helping however you genuinely can, perhaps offering general guidance about how cable cancellations typically work, addresses the underlying need without overpromising involvement in something outside your actual iptv panel business entirely.
This request usually comes from genuine overwhelm rather than unreasonable expectation, a customer juggling a significant life change, switching providers, and simply hoping someone else might handle one more piece of an already tedious process.
Acknowledging that frustration with empathy, even while declining to personally make the call yourself, tends to land better than a purely procedural refusal that doesn't address the genuine overwhelm behind the unusual request.
For an iptv service business, drawing this kind of boundary clearly but kindly protects your actual scope of responsibility while still showing genuine care for the customer's broader situation.