Trying to open specific ports between subnets on tomato 1.28 shibby.

[GawainXR]

Hello, I'm currently working with TomatoUSB 1.28 Shibby on an Asus RT-N66u

I have two subnets running on the router
(192.168.3.0/24 , BR0) for my personal stuff.
(192.168.4.0/24 , BR1) secondary subnet which is an isolated subnet that my roomates use.

I have Plex media server running which I'd like to share with my roomates, however since it is running on a server box that has other services on it, I want to ONLY allow specific ports as up posed to the host itself.

I have IGMP Proxy running and UPNP clients on the 2nd subnet can see the broadcast, they just can't connect due to the firewall.

The ports I'd like to open are 32400 TCP
1900 UDP
The address of the media server host is 192.168.3.21


I'm sort of familiar with the function of IPtables and i'm hoping there is an ip tables command to bridge those two ports on the 192.168.3.21 BR0 host, to all of BR1

Thanks for any help in advance!