OK, there are really two good solutions here. VPN is generally a bit slower given the transfer of data and more dependent on bandwidth.
If you want to keep the data on premises, then the best way would to be to go with TS/RD. Terminal Services, Remote Desktop. Or Remote Desktop Services. Microsoft has nothing better to do than to change nomenclature every year.
Part of the idea I am going to tell you requires certain hardware. Terminal services now must be in a domain, where back in the 2003 era, it could be separate and on a workgroup.
Basically, you have your home network, hopefully in a domain. It has to be in a domain. So, let's say you have your network with 10 PCs, and a server like Server 2008 R2. It will be much different if you are using 2012 with VMs or even some of the new VDI stuff, but I will let that go for later. So, you have your file/database server that runs SQL Server and other databases and shared folders. All of your client PCs are running Amazing Charts apps, and they all connect to the AC database on SQL server. Each of these PCs are being used so it would be rare or haphazard for users in your remote office to remote to them. And, then someone would need to use that computer. So, how can you share one app? Terminal Services on a Terminal Server.
So you will basically need a second server running a server OS. You can't use Win 7 Pro or Windows 10. These have hacks, but they aren't legal, and I wouldn't recommend going that way.
So, now you drop in a second server with its server OS. Regular PC OS do not have a role for TS. You go into the terminal server and enable the TS/RD role. You now drop in any app that is compatible with TS, which is most. I am pretty sure AC is. I would ask before I went to all this effort and expense. Now that the app is on the TS and terminal services is working, many users can access the app. Those users will each need a license to establish a remote connection. Or you can do it by computer, but better off doing it per user. So, now each user at your remote office has their PC or thin client (I would go with PCs) can use Remote Desktop Client to log into the terminal server and use the app. Each user will see the app as if it is on their own desktop. You can use other software during the same session.
There are also things called RemoteApps which are essentially the same as apps except that when you log in you see only that app filling your whole screen.
Now, unlike transferring data over the Internet, the only bandwidth you are using is for your mouse clicks, your keyboard and your the visualation of the monitor. In other words, almost nothing. So, your bandwidth is plenty.
Terminal services is the way to go, but there are some limitations as stated. You need servers and you have to have client/server domain network. Now, if you don't have this, and you are using a P2P, here is a good opportunity to upgrade to a domain.
The key thing is if you want to go this route, you will likely need to hire someone with IT experience preferably a professional. It's fairly straightforward, but not if you haven't done it before.
Of course, if you did upgrade to a server setup, you would likely get 2012, which runs Virtual Machines, one of which can work as your TS. Then there are VDIs. No need to go there right now. But, if you do have a server running 2012 already with HyperV, etc. then you are really already there.
But, say you did have to put out the expense of a server, etc., it would be worth it to set it up this way. Then, when you get another office, you just purchase more licenses, and you are good to go.
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Now, the second, far easier way to go is to run AC in the cloud. Amazing Charts hosts a cloud version, and Indy hosts a cloud version. Then, everything is done, and you can access AC from any office, Smartphone, home, Starbuck's, wherever you have Internet access. The cost is per user per connection, so it can get a little pricey. But, you don't have to worry about backups or updates, etc.
