I am using contao 4.5.8 . Can I use the root configuration " Assets URL " to load my assets from a cdn ? Any other configuration needed for my requirement ? I fount an issue belongs to TinyMCE ,is this fixed ? And I also need to know alternatives for rackspace .
Contao supports a Pull CDN via the Files URL & Assets URL in the website root configuration, yes. No further configuration on Contao's side is required. As the names suggest, the Files URL will be prepended to any URL pointing to the files directory, while the Assets URL will be prepended to any URL pointing to the assets directory.
On a side note: Contao 4.5 already reached the end of its life a few months ago, you should update to Contao 4.6.