Broadcast a private wireless network from Mac to iPhone via ethernet
On the Mac, go to System Preferences > Sharing. In the list on the left-hand side, check the box for Internet Sharing, then ensure your current connection (probably listed as Ethernet) is selected in the combobox. In the checklist below that, check the box for Wi-Fi.

(In the screenshot, Internet Sharing on the left is not checked, but it must be for this to work.)

The Wi-Fi Options button allows you to set the network name (SSID) and password.