I started a contract with telekom in Jan 2025, but then when the technicians came to check the DSL installation he said I can't use telekom in my building as something is wrong with the installation and needs to be checked by the building owner, so we had to cancel the contract same month, I have a confirmation of my canceled contract and the return of the device, yet I am still being charged till now. How can I follow up so they don't charge me anymore!