You're in a bit of a tough spot, not having a network. However, probably, your best bet would be to use Azure SQL Database (or AWS RDS SQL Server, your call, I'd lean towards Azure pretty heavily). This lets you put your database on the cloud in a Platform as a Service offering. This means, most of the administrative tasks are handled for you, from backups to consistency checks. It removes tons of headache from your life since you won't have a full time administrator to take care of your SQL Server instance.
Connecting to it through ODBC is easy enough to set up. You can still secure it so only certain people can get through. You'll still be able to use the Access front-end. It does require internet connectivity, but you're in a situation where that's just going to be part of the deal. You simply can't have distributed people accessing a central database without connectivity being part of the game.
Transition from Access to Azure SQL Database should be relatively painless (it won't be perfectly painless though, plan for a period of testing & validation).
You could also do this with cloud-hosted virtual machines, but the set up, installation and maintenance of the system this way puts a lot more knowledge and skill requirements on your organization. Azure (or AWS), offers the best way around the majority of those skill & knowledge requirements. Not all of 'em though. You are moving to a brave new world, so plan accordingly.