Connect MX21 Pro to your cable modem and power on both devices.
Log in to the Tenda setting interface (tendawifi.com) ,follow the directions to finish the setup steps.
For the other two MX21 Pro,just simply power on it to automatically join the network.
Tenda Technology Tower ,#1001 Int' l E-City Zhongshanyuan Road,Nanshan District, Shenzhen China 518052