PPTP server - Clients still have original IP on the same server

Question

I've configured Poptop (The PPTP Server for Linux) and it is working fine.

Clients are assigned different IP addresses which are visible publicly properly, e.g. on www.whatismyip.com.

But on the same server, all request from clients are recognized as coming from the original IP addresses.

How to make server, where pptdp is installed, see assigned IP addresses instead of original ones? I understand that somehow traffic should be routed out to the Internet and back, but not sure how.