iOS 27 moved this setting. Here's how to turn it off
iOS 27 moved Wi-Fi Assist into Wi-Fi settings and renamed it Connectivity Assist. If Wi-Fi Assist was on, this is on too, and it can use cellular data while you're on Wi-Fi.
- Open Settings > Wi-Fi and turn off Connectivity Assist.
- Or turn it off for one network with the (i) button.
Some users report better battery with it off. Spotty Wi-Fi? Keep it on.
Source: Apple Support