Fixed Internet has not been working for several days. What is the best way to record the fact of a complaint to the operator?

The further we go, the worse it becomes. Nos started as an incompetent and cheating provider (details are here).

And after 30 days since the contract started, Nos demonstrated that it has no quality of services as well.

The fixed internet stopped working in my flat on Thursday, 5/01. I have tried to fix it with the Nos app. The app stated that there was no fixed internet and tried to reboot the router and fix it remotely. I followed the app’s instructions, so it made a few attempts, but nothing was repaired. I was in a hurry, so I decided to try the next day again.

On Friday, 6/01, the fixed internet still wasn’t working. Again, I have tried to fix it with the Nos app. The app stated that I had troubles with fixed internet and started an automatic procedure. A few attempts were without a result (what a surprise!). Finally, the app gave up and arranged a call to a technician.
I explained to a technician that I didn’t have internet since 5/01, and he answered that he knew it. He also said that the earliest day to arrange a technician’s visit is 8/01. The technician also insisted that a three-day shortage of fixed internet was typical for Nos. So, they were not going to fix it faster and were not going to deduct it from the invoice. 

I pay for the fixed internet to be sure that I have a fast and reliable connection. And I need to be sure of it at any moment. I am dead sure that a three-day lack of services shouldn’t be typical in any circumstances. In case of an accident, the operator should find a way to deal with the problem fast, and while the client doesn’t have a fixed internet, offer alternatives to the client. E.g., make a mobile internet unlimited. And of course, the days without fixed internet should be deducted from the bill.

So, my question is: 
what is the correct and fast way to send Nos an official complaint?

If the problem occurs a few more times, I will complain to ANACOM and insist on contract termination due to not providing services and non-fulfillment of the terms of the contract by the operator without any penalties/compensation from the client. Hence, I want to ensure that my complaints to NOS are recorded correctly.