First check in Matterport Cloud that your Space is Public.
Next put the embed code on an HTML page just by itself (see below). If this loads then the <iframe> tag is fine and the problem is elsewhere.
<iframe width='853' height='480' src='https://my.matterport.com/show/?m=9dwWy8VsSw5' frameborder=0 allowfullscreen></iframe>
If the Space still doesn't load, try changing the embed code from https to http.