I have fixed it!!!!
I tried all those suggestions, changing date/time, reset phone setting, hard reboot, changing DNS, restore phone... each solution would fix the problem only temporary (at least for me!) and the bug still keep coming. Using cellular data to download the app in the beginning and switch back to wifi seems to help as well, but that's not convenient! I don't want to keep doing the same thing every time I download apps.
Here is how to fix it permanently (at least for me):
- Go to Settings/Cellular/ and turn off Wi-Fi Assist. That's all!!
- Sometimes you need to delete the apps that already caused the problem, before download them again on Wi-Fi with the Assist thing off.
You can switch it on later. But for me, once I switched it on, I was able to download a few apps, and then out of the blue, the problem came back! Then, I switched it off, the problem was gone. Also, be sure you delete the un-finish-downloaded apps before you download them again.
So, the story is that I have been using 4s for 3-4 years with none of this 'Unable to download app' problem. Then, once I used my Apple ID with my new SE phone (with the Wi-Fi Assist on). The problem occurred. Not only that, the bug also appear on my old 4s phone as well. That's very weird!! So, I thought that it has something to do with how the new phone communicate with App Store while downloading app. Now that I turn the Assisting thing off, my old phone is back to normal. 🙂
Hope this is helpful to you too!