5 Ghz and 2.4 Ghz problem, what I can do pls?

Hello everyone, so I'm really not good with everything related to internet etc that's why I need your help

I have a device that need wifi to work, but the huge problem that I didn't see coming is that he's not compatible with 5 Ghz and my router of course is 5 Ghz...
Then I had to change some settings of my router, I activated the 2.4 Ghz and desactivated the 5 Ghz but even after all this, my device show "Not compatible with 5 Ghz"

If I changed the 5 Ghz into 2.4 Ghz then why it's still not working? How can I resolve this?

Thanks for reading