Since iOS 18 FaceTime showing connection unstable

My Mom, my sister & I live at different locations. My mom & I are in the same state (again don’t live together) My sis lives in another state. I used to troubleshoot internet for a major telecom company so I know my way around a computer. I have Fios 1gb connection. I have 980 dl & 900 ul. My ping is 2ms.

I have a stable internet connection, can run multiple devices at once. My husband plays his Xbox & we use the firestick ( streaming) all at once.

since iOS 18 (I believe it to be the problem) FT will cut out on us & hang up. My sister or Mom or me will go to a light grey blurred out screen that says “CONNECTION UNSTABLE) it happens on all our screens). We are not all on FT at once mind you.

i have reset the router, made sure to update the router to the latest firmware. We were on with 2 Apple techs. We both (My sis & I ) deleted the FT app & reinstalled it again & went to settings> general>> reset>>> reset network settings on both the phones & iPads. FT STILL CUTS OUT

We have tried to turn off the WiFi on our cell phones & turn Bluetooth & the hotspot on & connect the iPads to our hotspot on the phones. FT STILL CUTS OUT.

I turned off my WiFi using only my 5g cellular data connection on my phone & called my sis. She answered on her iPad & a few minutes later FT STILL CUTS OUT

Clearly there is a problem here Apple 

Kindly fix this.