Enter these codes into the Dialer to enable the options in Settings. (Settings < SIM Cards & mobile networks < Sim Card Settings < VoLTE Enabled/Make Calls using WLAN)
Things should work after you enable the two options. This depends on whether or not your carrier supports it and your e911 address set (US only?).
Some issues you might run into is that the option for Network Preference is greyed out and you will unable to change that option. I've had some success if you disable it and re-enable it and click on that option really fast, it will go through. When you first enable VoWiFi, it is important that you get into this menu and change it to WiFi preferred or WiFi Calling won't work.
Manually creating your carrier APN worked great for me. Been testing it for about 2 weeks and I haven't had any issues with VoLTE/VoWiFi. For example, for me on T-Mobile, I created a new one with the following settings. The important part I'm guessing is the APN type. Also make sure that your carrier has enabled WiFi Calling on their end as well.
MMC : 310
APN type: default,supl,mms,ims,cbs
APN protocol: IPv4/IPv6
APN roaming protocol: IPv4/IPv6