Android Box not connect to internet

Hi,

I purchase 3 Android Tv Box with Kodi 16.0 and Android 6.0, dual band WiFi

2 out of the 3 units are not connecting to wifi only connecting successfully via Ethernet.

I have tried 2 different ac modems and one b/g/n modem I have adsl connection and none of the 2 boxes will connect. I have successfully connected one Box to 2.4ghz wifi and I cannot successfully get this to connect to 5.8ghz ac wifi as yet. It picks it up but will not connect.

Other 2 will pick up the signal but will not allow connection.

Hope someone can help

Thank you.