Given that you already have mp4 files, instead of trying to import the files to iMovie or rely upon the native Photos or Files App, perhaps consider using a third party player App - or a good quality File Manager App, some of which have excellent player functions built-in.
For a player, of the many available, consider VLC Media Player:
https://apps.apple.com/gb/app/vlc-media-player/id650377962
For a really good file manager that includes excellent player functions, I recommend Stratopherix FileBrowser - Document Manager, or its “pro version” FileBrowser Professional:
https://apps.apple.com/gb/app/filebrowser-document-manager/id364738545
https://apps.apple.com/gb/app/filebrowser-document-manager/id364738545
You might review the extensive online Manual and help files to determine suitability for your needs.
https://www.stratospherix.com/products/filebrowserprofessional/filebrowser-professional-user-guide.php
Once your mp4 files have been transferred to your iPad, you’ll be able to view them at your leisure - without need of a network connection or access to Cloud storage.
You can transfer your files between your computer and iPad in a variety of ways. See:
Transfer files between your PC and devices with iTunes - Apple Support
For Mac, you’ll need to use either iTunes or Finder - as available for your installed version of macOS. For a Windows PC, you you will need to install Apple iTunes and/or iCloud for Windows. You can install and use both on the PC - as they will happily coexist - and provide considerable flexibility in how you move/share/transfer/manage data, photos and other documents between iPad and Apple devices.
iTunes for Windows
Download and use iTunes for Windows – Apple Support
iCloud for Windows
Download iCloud for Windows – Apple Support
Set up and use iCloud for Windows – Apple Support
You might also consider simply copying files to a USB Thumb Drive (applicable for both Mac and PC) and using this to transfer the files to your iPad. If your iPad has a Lightning port, you’ll need an Apple Lightning to USB 3 Camera Adapter with which to connect the USB storage to your iPad:
https://store.apple.com/uk/xc/product/MK0W2ZM/A
