Why: Wi-Fi Assist Greyed Out (iPhone)
Wi-Fi Assist is iOS feature that automatically uses cellular data when Wi-Fi connectivity is poor. By default, Wi-Fi assist is enabled on your iPhone.
If you notice Wi-Fi Assist (Settings -> Cellular -> scroll down to bottom for Wi-Fi Assist) greyed out, it’s due to Cellular Data is disabled on your iPhone.
If you have Cellular Data off, Wi-Fi Assist cannot use LTE cellular Internet signal to connect to the Internet.