My IoT devices will not connect to this range extender nor will they connect to my RE715X, even though they will connect to my main unit, which is a BE550.  
 

All if my IoT devices will only connect to the main unit.  My non-IoT devices connect to the ranger extenders pretty well.  
 

Note:

The firmware listed above is for the main unit. (I don't know if it is different than the main unit.) Also, I don't know where to find the hardware version, so I just put V20.