I think you need to upgrade the Android version.
I had the best success for AC connectivity using Android 5.0.1, AC connects every time and upgrades work. I'd recommend this version. Google how to update Nexus 7 to 5.1.
I also use a Samsung S2 8" with Android 7.0 but it takes 2-3 attempt's to connect to the AC.
Phantom 4, latest firmware, latest DJI Go 4
I had the best success for AC connectivity using Android 5.0.1, AC connects every time and upgrades work. I'd recommend this version. Google how to update Nexus 7 to 5.1.
I also use a Samsung S2 8" with Android 7.0 but it takes 2-3 attempt's to connect to the AC.
Phantom 4, latest firmware, latest DJI Go 4