pretty sure you just add the list of networks that should be considered local to the plex settings in the admin page and it's done. Works for me.
> https://support.plex.tv/articles/200430283-network/
Scroll to "LAN Networks"
"Very few people will need to set or change this preference. It simply lets you specify which IP addresses or networks will be considered to be “local” to you..."
"Tip!: This feature requires an active Plex Pass subscription for the Plex Media Server admin Plex account. Addresses can be specified either as an individual IP address or a range (using IP/netmasks). Do not include spaces or tabs."