So at the bottom of the wifi / internet settings, when you click “Add network” and type in the SSID of your hidden network it isn’t found? This should be working, maybe you can clarify more the steps you are trying. Maybe you are specifying the wrong security format, for example?
Alright. I was wrong, I succeeded by setting the network manually AND selecting the “hidden network” choice on in the advanced settings.
Before that, I was joining by a QR-code, which works correctly with the other phones.