Why is the wifi network visible to everybody but me?

Recently my flatmate changed the maximum number of devices that can connect to our Wifi and since then I cannot see our Wifi network.

I am the only one in my household using Ubuntu. I logged into our rooter using a webbrowser and I can see other people's non-Ubuntu devices connected and working flawlessly. Their owners can see the wifi and can connect anytime. When I connect by cable, I can see my computer too, but it is not possible to connect wirelessly.

Here are the things I tried:

1. I checked that I am not blacklisted - I am not
2. I tried to "Connect to hidden Wifi network", entered SSID and password - no effect
3. I restarted the router multiple times - no effect

Please, what can be causing this and what can I do next?

EDIT:

I am using an Orange Livebox and here is a screenshot of the devices connected: