Hello  @LUISMR 

Thank you for the feedback, as far a I could tell AP mode does not change the MAC address.

• Please confirm you have setup range extender into AP mode as guide here correctly  

How to setup Access Point Mode of Range Extender?

• Check if your Laptop / mobile device has enabled Random MAC address, if so , you will need to turn that off on the network client itself.  (please check my pm for some third party article)To help isolate the issue, you could connect that client device directly to router and see if the decvice show the real MAC on router client list. 

If still the same, may I know what is the network device; and the network topology, did you connect router -- RE(AP mode) directly, or is there any other extender or router switch between AP and main router, can you share some photo of MAC showing the concern, thank you. 

  @LUISMR 

Thank you for sharing more case details. To help isolate the issue, please try by pass the switch and connect Router--TL-WA850RE (AP mode)directly, 

Can you help describe the network topology, and the model of wifi clients in details : e.g. Router Archer C7<Ethernet>850RE (AP mode) <Wireless>HP Win10 laptop,  iphone 7 IOS 14.etc

Can you share a photo of the MAC on router & a photo of on RE page, and real MAC of device as well, you could share the photos via Google driver or OneDrive and share a link of photos via private messege, thank you.