Echo while Zooming

Hi.
I'm using Zoom simultaneously, in my computer and in my smartphone. I'm teaching students how to use the applications. During the meeting, I share my screen from my smartphone, while I'm also attached to the meeting by my computer.
There is a strong Echo while I speak. In some cases it can change into feedback.
I turned off my zoom microphone, also unchecked the smartphone's microphone. Also put down the computer's speakers and tried to hear from my smartphone only. It didn't get better, the echo remained.
Is there a way, I can do in order to make things better?
Hope there is...
Waiting for your kindley reply and help.
Thanks,
Boaz