iPhone 7 Plus Keeps Using Cellular for FaceTime

[kr4yzi3]

Not sure if anyone has experienced this issue before but our iPhone 7 Plus keep disconnecting from Wifi and switch to Cellular whenever we FaceTiming. The symptom is:

• If we're both on Wifi with Cellular off, our call will last for a few minutes before showing Poor Connection, then Reconnecting, then Failed. I need to mention that our connection is strong enough that images are crystal clear.
• If I have both Wifi and Cellular on, our call will last for a few minutes and my phone will immediately switch to Cellular. The image quality will be downgraded a little but acceptable. I checked the Wifi settings during this time and everything is there (IP address, DNS, etc...). If I turn off Cellular, we're getting disconnected immediately.
• If I turn on Low Data Mode, our call is not dropped anymore but our call quality takes a hard hit.

So my theory is could it be because the hardware limitation that highest quality call cause the phone to disconnect from Wifi? It doesn't sound right to me though. Both of our iPhone 7 Plus are on iOS 13.

I'm willing to switch to some other services for the call but just wanna see if there is any other solution.