Static IPs on PPTP server on Balance 20

I have computers connect to my peplink via the pptp server, and I’d like them to be given a status IP (can be based on user as I have one user per-device).

Saves me having to register the IP every time in a ddns.

Like for download? But when the packet reaches Peplink, it has already traveled the difficult journey. From Peplink WAN interface to Peplink LAN interface is easy, the difficult part is from where it was when it started coming to Peplink WAN through the Internet.
Not sure if QoS for download would help much