@Helle987 Hello, 

Please double-check the 2.4Ghz wifi password of your router when set up RE again. 

Please verify your 2.4Ghz Wi-Fi password by forgetting your Wi-Fi network on cell phone first :  
Go to Settings-> WiFi on your phone, Tap (or Long press  the router's WiFi name  ->Forget / Delete the network 
Please reconnect to your router wifi and it will ask for a password, will it accept your router password?
 

If still does not work, you could change router's 2.4Ghz WIFI channel to one of the wifi channel among 1-11 and see if that works.

If that does not help, may I know the model of router & RE, and a photo of the 2.4Ghz wireless settings?

@Solla-topee 

thank you so much for your suggestions, I've doublechecked the password and I reset it twice since and still the 2.4 is still not working...

any other suggestions? 
 

Kindest regarded 

helle 

@Helle987 Hello, 

Thank you for your update, may I know the model of router & RE? 

Do you mean RE could detect router's 2.4Ghz but failed to connect to that 2.4Ghz or it does not see router 2.4Ghz at all? 

You could contact router support and change router's 2.4Ghz Wi-Fi channel to one of the wifi channel among 1-11 and see if that works.

If still does not help, may I know your network topology : for example router  model )))(( RE )))(( laptop 

Please also check the firmware & hardware version of your extender and let us know, thank you. 

https://www.tp-link.com/support/faq/315/#2