(Sadly as living in Spain for about a year I’m still not in such place to raise this or understand the full steps needed)
Used my digital certificate (which is installed in the browser), but AFAIK, you can use Cl@ve on that page above too.
In the past, I've cited BOE-A-2022-10757 (https://www.boe.es/buscar/act.php?id=BOE-A-2022-10757), done a reclamació for the repeated loss of lawful access on my connection, and a denúncia about a broader overblocking practice affecting access to lawful services.
Also, supposedly, we should be able to make claims to CNMC as well, but haven't figured out how. Also of course, been complaining to my ISP every time it happens too.
We've never guaranteed the right to free speech and because we haven't it's a slippery slope all the way back down to the furnaces of autocracy we sprang from.
The Spanish president has come out on record saying we don't deserve anonymity on the internet.