It’s settings / cellular (at least on my Verizon phone running iOS 11.4.1).
I have it at Settings > Phone > Wi-Fi Calling but since I’m with T-Mobile it also gets mirrored to Settings > Cellular > T-Mobile > Wi-Fi Calling. This is on iOS 11.4.1.
Sorry. There’s another submenu. Settings->Phone->WiFi Calling. There’s a “WiFi Calling on this Phone” switch. This is iOS 11.4.
Earlier versions of iOS will have this switch in different places. It may also differ based upon the phone.
The most useful step-by-step is this T-Mobile Guide. It gives you a selection of models and iOS versions. It should be the same steps as on Verizon and AT&T.
See if that helps. According to Verizon’s support pages, you can setup e911 via their webpage and if you don’t have it setup, you can set it up when you turn on WiFi calling.