Those of us old enough to remember the joys of traveling with a Floppy Disc in our bag have mixed emotions about new cloud storage options. If you don't have wi-fi, you have to be careful to find a solution that will allow you to edit the manuscript offline and will later sync back to the cloud later. However, online options are also less likely to fill up as quickly or die if exposed to sunlight (long story).
There are three main online options that are convenient and free to use.
Microsoft OneDrive. If you already have a subscription to Microsoft Word (or Office 365), then you can store your documents in this folder and access it from any device. It's one solution that you may already have available without having to get any new subscription products, so that may be your first bet. The good news is that if you have Windows 10 and created a Microsoft account upon sign in, you already have one probably available in your Windows Explorer folder.
Google Drive. If you already have a ubiquitous Gmail account, Google Drive is already ready for you to use! The free version starts with 15 GB of free online storage, which is plenty if you are writing a manuscript using common tools, such as Microsoft Word or Google Docs.
Dropbox. Dropbox is another easy to share and store files, with 2 GB of storage for your use. You can sign up for the paid version for more space if you need some.