Hi Ralf,
What is the method you're trying to for this import? It's not clear where such a limit could come in, from your description. (PHP file upload limits aren't relevant to the normal way of restoring in UpdraftPlus).
If you have a copy of the database backup, then you can place it manually, by any method (e.g. use FTP) in wp-content/updraft, and then press the "Rescan local storage" link in the "Existing Backups" tab. Then UpdraftPlus will detect the backup, and you can restore it. Or, use the drag/drop uploader in that same tab, instead of FTP. Neither of these methods is subject to any file upload limit.
Or are you saying that your web hosting provider doesn't allow any files whatsoever on its filesystem that are larger than 20Mb?
Best wishes,
David