How Can I make it so My guest WiFi On my AR750s Can only Access My Plex server only or smb Shares and not have Internet access.
I haven’t been able to test any of this yet, however I think if you use luci to do the following in the firewall rules may do what you want (or at least get you close)
If you edit the GuestZone and remove the forwarding the WAN zone, that should disable internet connectivity.
In traffic rules you would need to create a rule to allow from guest zone to lan zone ports 32400 (Plex) and ports 139, 445 (SMB).
You may also need to forward to the LAN zone in the Guest Zone.