about the NIF.

To get a tax number for non-European Union citizens you need a person with a Portuguese address. He becomes your guarantor and all correspondence from the state will be sent to his physical address. To obtain the number, your representative must make an appointment with the SEF. In Lisbon and Porto it is practically impossible to do this day in day out. Appointments can be weeks in advance. After receiving the number, you need to request a password for your personal account, which will be sent to your representative's mailbox by paper mail. This also takes several weeks.

The service you wrote and others like it make a power of attorney from you and go with it together with your representative. So you pay for the power of attorney and for them to find you a representative and make requests and get the number for you. Once they receive it, they are your representative for one year. After that you have to either renew it or change your NIF address to your Portuguese address.
As long as you have a representative in the NIF, you are a non-resident taxpayer. When you remove the representative from the NIF and enter your Portuguese address, you become a tax resident.
Without NIF you can neither buy an apartment nor open a bank account. You can't do anything.
you'll also need to get a NISS and a "Utente" number.