How can I to manually assign WAN IP?

Server Version#: 1.18.8.2527
Player Version#:

Somehow, My Plex server is running behind the home router & a second proxy server. the structure below.
plex server(LAN IP) -> Home Router(WAN IP A) -> proxy Server (WAN IP B).
right now my plex server always get the WAN IP B only. How can I manually assign correct WAN IP, the WAN IP of my home router?

many thanks for your help in advance!

no one know about it?