Long video is now supported in latest iOS musical.ly. You can upload a long video by following steps:
1. Open musical.ly and tap the "yellow plus" icon.
2. Tap "from library" and select "long video (beta)".
3. Choose a video and add a caption.
Please note that maximum time limit of a long video is up to 5 minutes, and only videos within 5 minutes will be shown from library.