You can do it locally on your mac without changing your router using pf anchors.
In PIA - Look at directions to enable port forwarding (must use assigned servers).
On your Mac:
Using terminal create a file in /etc/pf.anchors/plex.pia. run the following command:
sudo nano /etc/pf.anchors/plex.pia
in nano, copy the following:
#replace MYPORT, with the PIA forwarded port
#replace en0 with whatever your local interface is
#replace utun1 with whatever interface PIA is mapped to. Use ifconfig if you dont know
rdr pass on en0 inet proto tcp from any to any port MYPORT -> 127.0.0.1 port 32400
rdr pass on utun1 inet proto tcp from any to any port MYPORT -> 127.0.0.1 port 32400
Create a pf config file by using the following command:
sudo nano /etc/pf.plex.conf
Copy this into it:
load anchor “my-plex” from “/etc/pf.anchors/my.plex.vpn.forward”
Start pf by running:
sudo pfctl -ef /etc/pf.plex.conf
Go to settings -> server -> remote access and manually put in whatever your PIA forwarded port is (same as MYPORT above).
Hope this helps.