The setting you're looking for is called Connectivity Assist in iOS 27 — it used to be Wi-Fi Assist, and Apple moved it. On iOS 27 / iPadOS 27 it lives in Settings → Wi-Fi → Connectivity Assist, with both a global toggle and a per-network option. On iOS 26 and earlier it's Settings → Cellular (or Mobile Data) → Wi-Fi Assist.
What it does: when Wi-Fi is weak, Connectivity Assist combines Wi-Fi and cellular to keep the connection usable — which can mean meaningful cellular data use even though you're 'on Wi-Fi'. Apple documents exceptions: it isn't used during international roaming, while cellular Low Data Mode is on, or with some third-party apps that stream audio or video.
To keep the phone Wi-Fi-only: turn Connectivity Assist off globally (or for the network in question). The phone then stays on Wi-Fi and doesn't quietly fall back to cellular. If you'd been seeing surprise data drain, check the cellular usage view afterwards to confirm the change took effect.
Sources: https://support.apple.com/127686
— Osnivo Support