@help4me 

Log into your Arris router and look at the 2.4GHz password to make sure the right one is being used.

Make sure the Arris wireless network names (2.4GHz and 5GHz) are different and not the same. Default the extender by holding the reset pinhole while its plugged in for 10 seconds. Use the Tether app to set up the device.

Once the setup is completely the 2.4GHz, 5GHz, and circular LED should be lit.

Hi,

I recently bought a TP Link extender RE650 and also encounter the same problem as the user above.  

Able to log into 5GHz but not 2.4GHz.

My router is ASUS RT AC5300

My ASUS router SSID is name as ASUS_24 and ASUS_50 so I presume the SSID name should not be a problem.
 

I have reset the RE650 various times, setup using tether app as well as via computer but the problem persist.

Please advice what can I do ???

Try to specify a wireless channel in the router. I've seen that on Pace routers where the extender will not connect if the wireless band channel is set to auto rather then a specific channel.