I'm not sure if this issue is related to the Wifi Assist option, but I was having trouble in the early 9.0 release with the T-Mobile WiFi Calling option. If Wifi went down or I got out of range, then the iPhone 6+ failed to automatically shift to cellular for data connections through apps like Safari. I know you have to allow apps to use cellular for data, but the apps I allowed, especially Safari, would not connect to the internet via my cellular until I restarted the iPhone once or twice. It was annoying as my WiFi modem was going out and I needed to get info from the internet. I never noticed that issue under iOS 8.