Dual band wifi doesn't connect on Androids with OS lower than 7.0

At the hostel, we have a wifi device outside our room. It emits both 5ghz and 2.4ghz wifi frequencies at the sametime.
The problem is that any Android phone with Android version lesser than 7.0 can't connect to the wifi while those with Android 7.0 easily connect to it. I can't understand why.
Is there a solution for this or we will have to change our cellphones.
Note: All iPhones are also unable to connect.