One solution, I think should work
Have a NAS server connected to a internet box or having internet connection possible
ex: Synology NAS + DSM interface (I have a 703+)
Install/customize the VPN PPTP server with No MPPE (Injoy VPN client allow PPTP connection without encryption 'MPPE")
Dynamic ip@ e.g. 10.0.0.0
....
Authentification MS-CHAP v2
Manual DNS (e.g. your internet box ip@)
Define autorized users with needed access under the NAS.
Once done, the VNP client using PPTP + userid + password should be able to establish a connection after updating the internet box routing table
On you internet box open needed ports from from internet to your NAS ip@ to access the VPN server
Under VPN client, define the VPN server ip@ seen from internet and use PPTP as well + userid + password
I could not give it a test but I used similar configuration with windows (PPTP) and Injoy allows PPTP VPN (no encryption)
Not easy and of course, no supported encryption (MPPE)