Maybe this?

https://faberlibertatis.org/wiki/Hotspot_Debian_Sarge_HOWTO

# iptables -t nat -A PREROUTING -i tun0 -p tcp -s 192.168.2.0/24 --dport 80 -j REDIRECT --to-destination 127.0.0.1:3128

Maybe you should to enable STP on each router?