Hi @Thoreau,
In order to get to the screen you describe, you would have already tapped Republic from the previous SIMs screen. In any event, from the screen with Republic titled at the top, scroll down until you see WiFi calling, then proceed from step 6.
For the cellular data issues, I suggest resetting the APNs to defaults as described here: https://forums.rwusers.com/t/how-to-reset-an-android-phones-apns-to-defaults