I didn't see a way to turn OFF Cellular, but leave WiFi on...
That can be accomplished with this two-step procedure:
1, Turn on Airplane Mode. (This shuts down all 3 radios: cellular, Wi-Fi, and Bluetooth.)
2. Turn Wi-Fi on.
Curiously, the device will still show as being in Airplane mode, but Wi-Fi will work. Also will save battery time as the cellular and BT radios will be OFF.
- -