Plex cannot connect securely. Go to https://app.plex.tv/ > Settings Server shows as “Cannot connect securely”
This is due to DNS rebinding protections.
Plex’s article https://support.plex.tv/articles/206225077-how-to-use-secure-server-connections/ had a “pfSense” section; I searched for an opnSense solution and found: https://www.reddit.com/r/OPNsenseFirewall/comments/xwjxe0/unbound_dns_rebind_for_plexdirect_not_working/izztf7m/.
On OPNSense 22.7.9, I navigated to Services > Unbound DNS > Advanced.
I scrolled down to “Private Domains”
I added plex.direct to “Private Domains”
I hit Apply, then restarted the service.
Next I reloaded the Plex Web App in my browser, and boom - direct secure connection with the playback quality I expected on a LAN!