It's also easy enough to set up your own if you your router has an external IP address of any kind. If your router has a dynamic address you'll have to go subscribe to a dynamic DNS service, but there are free ones of those too. If you don't have an external address for your home router then you can set up an onion service instead. That's slower but works through all kinds of obstacles.
The basic steps for the basic service are:
1. set up a machine with an SSH/SFTP server, e.g. RPi or Beaglebone with some USB storage
2. enable SSH keys and turn off password authentication
3. give the machine a fixed LAN address
4. forward a port from your router to your SFTP machine
5. log in over the Internet via your favorite SFTP client: sftp://you@router.example.com/
Since you're on Ubuntu you won't even need special software to log in with. You can just connect with your file manager and drag and drop:
https://www.youtube.com/watch?v=9S4DV1PluzA
It's far more secure and private than any third-party "cloud" services. If you are providing file hosting for OS X users, there is CyberDuck or FileZilla. If you are hosting for legacy OS users, there is WinSCP or FileZilla for them.
