I’m checking with Mux to see if they support cloud conversion of MP4 to WebM (didn’t find anything in their docs). In the meantime, though, you can always do the conversion on your own and upload the WebM to Dato. Example using ffmpeg:
ffmpeg -i input.mp4 -c:v libvpx -c:a libopus output.webm
Then you can upload that to Dato and serve it directly from datocms-assets.com (or your frontend host/own CDN).
EDIT: Changed the ffmpeg codec to VP8 instead of AV1, due to limited hardware support on Apple devices.