At last, problem solved. I don't know if it is a coincidence but doing the following solved the problem. I factory reset the router from the UI. Then, (hard) factory reset with the reset button. Then...

@Kevin_Z Hi. Yes is an EU V5 version. The firmware was downloaded from the italian site (link): https://static.tp-link.com/2020/202012/20201222/Archer%20C7(EU)_V5_201120.zip No, there are no error...

@Ivaylo nothing, same behavior on all the three. contacted support, let's wait if they let me know something. btw, tried with a friend's C7 with an old FW, all works fine. Dunno what changed with...

Nevermind, my bad, got the form. It took me aoutomatically to the Italian site, things are a little different, @Ivaylo thanks for the reply, where I can contact the tech support?

@Ivaylo sorry, I was able only today to do some test. Anyway, resetting to factory default didn't do nothing. Printer, skyq-decoder and xbox360 don't connect to wifi 2.4 or 5G. I repeat, they work...

yes, those are my exact settings. well, i'll try a factory reset as soon i go home. thanks for the reply