I was experiencing the same issue that you noted in Nov, 2017. However, after trying many different techniques and tuning, I did find a simple fix that resolved the silent disconnect issue I know a lot of engineers are facing.
If you come across an iOS device silently disconnecting from your wireless, please try the following. It's a simple fix and quite honestly, resolved many of the iOS issues customers were facing.
Join Wi-Fi networks automatically
Your iPhone, iPad, and iPod touch can remember network and login information so that you can automatically reconnect to that network when you're in range.
If your device doesn't automatically join a captive Wi-Fi network, follow these steps:
Tap Settings > Wi-Fi.
Tap next to the network name.
Make sure that Auto-Join is on.
If you don't want to connect automatically, turn off Auto-Join. To see the Welcome screen the next time that you connect to the network, turn off Auto-Login.
... View more