Q) Router port forward for itself

Hi

I use ac68p with merlin firmware.

I want to use internal 80 port for web admin, and external 8000 port for web.

I've set port forward external 8000 to internal 192.168.1.1 80 but it does not work

So I did search iptables command, but it just open port.
# iptables -I INPUT -p tcp --destination-port 80 -j ACCEPT

So my question is, what can I do for use different port external, internal in router?