Sanitize iptables

Hello,
i have a problem with my router when i want to insert new iptable rules to drop something it often just not works or not works as intedet.

like insert iptables -I FORWARD -i br0 -o vlan2 -j DROP

or iptables -I FORWARD -i br -o vlan -j DROP

Does not seem to do much to stop traffic when the vpn is offline.

I think there might be a ACCEPT rule in the iptables that overwrites it.
How can i make sure that there is nothing that prevent new rules from working?