VoLTE not working

When I try to enable VoLTE on my iphone 8 plus it just keeps spinning and dosnt change. I keep getting notifications from straight talk that my phone isnt compatible anymore because it dosnt support 4LTE, and from what research I have done, moving forward all phones need 4LTE because they will need to use VoLTE to call and text, and from everything I read my phone should support these features, but I am unable to turn on VoLTE for some reason.