Why is my wireless network not recognized by the smartphone?

I've been using the EasyBoy 804 for a few months now. It all worked fine so far, but a few days ago the connection to my smartphone broke down. It is a Huawei P8. I have tried everything possible (reset the router and reset to factory settings on my smartphone.

The router can't be in my opinion, because all other phones and our PC and laptop have an intact wireless connection. When I turn on my WLAN in a smartphone only the EasyBox is not displayed to me, the routers of the neighbors already. With network add nothing worked out.

I would be grateful for any help.

Is your network running at 5Ghz and your phone just has a 2.4Ghz receiver?

Is the router dual-network capable (2.4Ghz + 5Ghz parallel)?

Can you connect to other Wi-Fi networks?

Does it matter, because it has always worked and now it is suddenly gone. And the problem can't be the receiver right?