@David-TP I solved it in a very strange way. I'll tell you the whole story, just in case it's of some value for your team.

Desperate as I was, I tried a couple things. First I turned on my main router's Wifi and set a very simple password to test. The raspberry connected, no problem.

After that test, I set up a guest network in the Deco app. No password at all, no security, all open. The raspberry connected there too.

So I was back to the main Deco connection and failed to connect. I checked several times every single thing and then I thought: ok, what if my password (what was something comparable to MonkeyIsland@4321) is the problem? So I changed it to a simplier one, just MonkeyIsland. And it connected flawlessly.

The curious thing is that the long password works on my router, but not with the Deco for just specific pieces of hardware. Why? Don't know. Laptops connect well, consoles too, smartphones, everything. But for some reason, raspberry (any software you install on it so I'd say it's Linux in general) fails to connect to the Deco when the password is like that.

Weird? Sure.

Thanks for your help anyway!