Iptables not routing guest network through VPN

I'm trying to have a guest network that is completely routed through VPN, yet when I change the iptables to do so, the guest network loses all access to the internet,

I am using shibby v117, Asus RT-AC66U and the guest network is br2. In this example, both br1 and br2 should go through the VPN, according to these lines:

iptables -t mangle -A PREROUTING -i br0 -j MARK --set-mark 0
iptables -t mangle -A PREROUTING -i br2 -j MARK --set-mark 0

But what happens is that br0 will go through the VPN...

Iptables not routing guest network through VPN