There are several reasons why you cannot access the Web UI :

• The main Mesh unit is not connected to the Ethernet Network of your internet box via a RJ45 cable.
• Your device is not connect the Wi-Fi network of the mesh unit.
• You are not entering the right credentials, by default those are "admin" and "admin" for the Username and the Password. If you changed it and don't remember it, you can reset your Mesh unit to its default factory settings.
• Your device is not update and you need to reset it to its factory settings.
  Please try to reinstall all your devices by referring to the following procedure : How do I configure my Wi-Fi mesh network?
  If the devices are not working after following this procedure please reset them by referring to the following procedure : How do I reset my Mesh units to the factory settings?