You would need to download your video from the My Videos page to your mobile device and use the Instagram app to post that video.
If you have an iPhone and need assistance saving a video to your smartphone, click here.
If you need assistance posting a video with the Instagram app, click here.
When is support available?
Read this link for support hours: https://support.wavve.co/en/articles/5373873-when-is-support-available-to-help-me