I am an American living in Porto and set up a bank account with an address in the UK per the advice in this article, and was turned down for my NIF today. I was told that the address has to be on an official government document and that bank statements do not work. I’m not sure if it’s different from place to place, but can 100% confirm that if you’re a non-EEA national living in Cedofeita in Porto, you will definitely need a tax representative, at least as of March 2019.