This is an annoying but serious problem, as evidenced by the hundreds of thousands of hits on the web regarding this issue.
The issue is as follows: You have a smart device (e.g. a smart light bulb, smart TV, smart thermostat, smart security camera, smart dimmer switch, smart plug, smart HVAC system, etc.) that only connects to a 2.4Ghz WiFi network, but your router is dual-band (does both 2.4Ghz/5Ghz), and your setup device, typically your mobile phone, auto-selects the 5Ghz band. In this situation, you can't set up the smart device.
To set up the smart device, you'll need: (a) a mobile phone that can provide a 2.4Ghz hotspot WiFi (this is typical), (b) a setup device such as another mobile phone or tablet. Then the solution is as follows:
Note: While this looks like a lot of steps, it is safest, because the WiFi settings of your other smart devices may get confused by the WiFi switching. Depending on your other smart devices, things may work without the first and last steps.