"Unable to download app"

Ok, thanks. Sounds like you only have a few other options to try.

1. Reset All Settings (does not delete any data)
2. Erase All Content and Settings (this will delete everything from the iPhone except the default apps. Basically this will take it back to its original default factory settings.
3. Restore the phone. Even if you have done this step, you may want to try again.
4. Recovery Mode

   If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support

5. DFU Mode

   How to put your iPhone or iPad into DFU mode - iMore

Try the following steps to fix the problem. After each step, check whether you are still having this problem.

1. Simply double tap the icon of the app that you are unable to download and wait for a few seconds. Your download should resume.
2. Tap Settings > General > Date and Time > Set Automatically > and slide to turn off “Set Automatically”. Manually set the date one year ahead. Now try to re-download the apps. If you get an error message, go back to turn on “Set Automatically” then try to download the apps again.
3. Restart your iOS device. Simply press and hold the on/off (sleep/wake) button until you see the red slider and slide the slider to turn your device off. To turn back on, press and hold the on/off button until the Apple logo appears.
4. Settings > iTunes & App Store > Your Apple ID > Sign Out. Restart your device, then Settings>iTunes & App Store>Sign in and then try again.
5. Tap Settings > Wi-Fi and turn Wi-Fi off and then on again.
6. Reset Network Settings by tapping Settings > General > Reset > Reset Network Settings, and then your iOS device will restart. Note that will reset all network settings including your Wi-Fi passwords.
7. Reset your iOS device by pressing and holding the sleep/wake button and the Home button together until you see the Apple logo.

So ... you went through the first three steps? How about the remaining ones that I provided you?