If the IKEA TRÅDFRI devices are not connected, please follow the following steps and check after each step whether the fault has been corrected:

1.  Closing and reopening the Home app
   
   
2.  Restarting Wi-Fi on the smartphone or tablet
   
   
3.  Restarting the IKEA TRÅDFRI Gateway
   
   
4.  Disconnection and new connection on the Home app
   
   
5.  Restarting the Internet-Box
   
   
6.  Resetting TRÅDFRI lamps and re-matching with the IKEA TRÅDFRI and Home app
   
   

Here's how to reset your TRÅDFRI:

Lamps:
press the main switch 6 times

Wireless remote control/motion detector/variable intensity control:
Press the connection button 4 times within 5 seconds.

Gateway:
Open the Gateway cover and remove it. Push a needle into the small hole at the top of the Gateway and hold it down for at least 5 seconds until the LED indicator no longer flashes.

Translated with www.DeepL.com/Translator

Manufacturer help for IKEA TRÅDFRI