That's pretty odd.
Try uploading to youtube?
Edit, also maybe this will help?
https://vimeo.zendesk.com/hc/en-us/articles/224983008-Setting-default-quality-for-embedded-videos
I fell for my own trick........ It doesn't work because the other section is visible on mobile, for the video background.
This will work. Replace this line:
#block-yui_3_17_2_1_1600516236805_10107 h3 {
With this:
#block-a89aabdb6864ee5ef820 h3 {