After plug-in your first mesh unit and connecting it to the router it usually takes up to two minutes for the router to identify the WAN/LAN IP address and connect to it. Once the mesh unit is ready its LED will be blue. 
Then, plug-in the second mesh unit in a different room, the LED will be red. You must press the WPS of the first mesh unit for 5 seconds and then the WPS button of the second mesh unit for 5 seconds. Wait for approximately one minute for the devices to pair. Once the devices are paired the LEDs are blue.  

• If the pairing process does not work, please reset the two units to their factory settings by pressing the reset button of each unit. 
Please refer to the following FAQ to see how to reset your mesh network:  How do I reset my Mesh units to the factory settings? 

• If the LED is still red on the first mesh unit, please check that the RJ45 cable is connected to the right Ethernet port: WAN port (mesh unit). If you put the cable in one of the LAN ports the mesh unit will not be able to retrieve the information of the main internet network, so it will not be connected to the internet. 
Please refer to the following FAQ to see how to configure your mesh network:  How can I add new Mesh units to the existing network I just configured?