The new build [18.6.4.688] solves the video problem [on Grandstream GDS3710 doorphone]. I was encountering the same no video issue with incoming calls from the doorbell, however found that the problem is using ring groups from the doorbell to ring simultaneously several extensions. When answering a call for a ring group, the mobile app doesn't show the enable video dialogue box. If I have it call individual extensions, then I get the dialogue box and video works.
If I wait a while to activate the video, the video options seem to freeze even after hanging up. To re-enable it I have to force-quit the app and restart it.