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Any suggestions? Because we tried using D.Tube and it doesn't work at all. So we don't have much options where to store the files. At least YouTube guarantees that these videos will stay online for years.

can't you just create your own ipfs node (vps for a few bucks/month)? You can then upload all videos on ipfs and share the ipfs.io link on steemit

Then buffering often takes a while till the video has been distributed through the IPFS network after the video goes online and first people are trying to watch it.