14 DAY TRIAL // According to a thread on Apple's Support Communities website that racked up 190 replies until the moment this article was published, Apple's iOS 9 does not allow turning on the mobile data for new apps on some devices.
This is another serious issue affecting iOS 9 besides the Wi-Fi connectivity issues we've also reported about and provided fixes for just a few days ago.
As described by users that have contacted Apple's tech support about this issue, Apple representatives don't know a fix for this bug which means that a fix will most probably be included within a future iOS 9 update.
The only thing you can do to make sure that the mobile data can be enabled for any application installed on your iOS device is to downgrade from iOS 9 or iOS 9.0.1 to the iOS 8.4.1 release that did not come with any mobile data or Wi-Fi issues.
The downgrade process is not very complicated, and you should have an iPhone or iPad running iOS 8.4.1 in just four simple steps.
Step 1 - Download iOS 8.4.1 IPSW
You will first have to download the iPSW file for your device from the IPSW Downloads website.
Step 2 - Enter DFU mode
Connect your iOS device to your Mac, hold the Home and Power for about 10 seconds and then release the Power button and keep holding down Home until iTunes says it detected an iOS device in recovery mode.
Step 3 - Restore using the IPSW file
Launch iTunes and click the "Restore" button while holding OPTION on your keyboard to be able to select the IPSW file you've downloaded.
Step 4 - Set up your new device
After the downgrade process has ended, you just have to set up your iOS device using the setup screen displayed on all new devices.
If you have a backup you've made for your device while it was running iOS 8.4.1, you can restore it using iTunes, but you will not be able to restore any backups you've made while the device was running iOS 9.
If you know any other way to enable mobile data for newly installed apps on iOS 9 and iOS 9.0.1, share it with the rest of the readers in the comments section below.