Hi fraumo,
I see at least two issues.
-First you need to set your router. As the pptp server is behind it you need to forward its tcp port (1723). But your router must be also "PPTP passthrough " more detail here:
http://think-like-a-computer.com/2011/07/28/pptp-passthrough/So after this step your pptp server is available from a remote IP by using your DDNS. You can test it via a smartphone for example.
-But in many case, on the workplace you are behind a proxy server and if its administrators do not open the port 1723 and block GRE protocol, you won't be able to connect to pptp server.
(note: if GRE protocol are supported but port 1723 not opened, you can try to use 443. So you need to cross this port on your router. You may not be able to use the port 80 as the proxy server should not support encrypted requests on this channel if the admins have rightly set the proxy server)
If you can connect you to the pptp server, then you can use any local ip get from the vpn server (nas or other hosts)