Internet subscription cancelation

Hello, 

I have been unable to reach anyone by phone but I am leaving the country next week and need to cancel my internet subscription. 

What do I need to provide as proof that I am leaving the country? I will deregister with the Luxembourg authorities on Monday next week. Will a notice from them suffice? (I will not be able to register with my new country of residence for another two weeks and I need to return my modem before then). 

Please let me know how else I can reach a representative (all emails I receive from Customer Support come from no-reply addresses!). 

Thank you