when trying to upload (replace) world on a Minecraft realm the upload states 0MB of 0MB. After a few minutes the upload of 0MB finishes and then brought to a screen saying “Initializing download” with loading bars. After waiting 20+ minutes this stops and says upload failed 

note: World size around 300MB

Edit:

I was experiencing this on my Xbox one S. however it did manage to successfully upload on my computer.

This could either be a system problem, or a network issue on my end ( Low Speeds )