WTF I AM DOING WRONG (VPN edition) ?
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I agree with Scott here - You shouldn't use your Windows server at all for the VPN solution.
What does your Windows server do for you? If you're not using it for Active Directory, and it sounds like you're not using it for file storage either (You have a NAS), then what? An application server? Are you hosting your websites from it? Seem expensive for no reason, unless an application you purchased required the use of IIS - then I'd ask, can you get rid of that and move to a solution that is uses a free OS.
As for VPN - If you really need traditional VPN, Find out if your current router/firewall can do it. If not, replace it with a EdgeRouter. They are very inexpensive and do this job great, and work with the native VPN client inside Windows PCs.