iPhone 11 Pro FaceTime Calling

I got an iphone 11 a week ago and have been having this same issue. Whenever I facetime my wifi drops and it goes to LTE! As soon as I’m done with the FaceTime call, WiFi picks right back up! I tried turning off Wi-Fi assist. I made a FaceTime call, wifi stayed on for about 30 seconds and then changed over to LTE. So now why wont my wifi stay on during facetime??! I have a very fast internet. I speed tested itinerary the phone It showed 310 download and 394 upload. So now why wont my wifi stay on during facetime??!