@Dan_Danny I used no special programs, I just turned on and off mac filtering on both the router and the extender systematically to find all mac addresses of connected devices. It took a while......

@jgm I just bought the RE205 to extend my Archer C9. I had the same problems as you and almost gave up. I managed to find all the seven (7!) MAC addresses for the RE205 that needed to be added to the...