Address for Customer Services

I cancelled my internet/tv/phone after 12 days due to download speed being cut to 1mbps because I had used 80GBs, even though I was sold an unlimited package. Nos have said this is not true, even though it was their technical department that told me. They are charging me €681 for breaking contract. The letter from them has no address. I need an address to send all proof of speed tests, phone calls etc.
I used the "livro de reclamaçōes" in the Nos shop, but Customer Services just dismissed my complaint without any evidence.
I want to send a registered letter, but I can not find where to send it.