RT-AC66U doesn't provide internet access to LAN devices, Shibby

Hi all.

For a long time, I have used my Asus RT AC66U as a wireless access point connected to a stock ISP router.
It has a Shibby firmware, couple of years old. Maybe 135.
This weekend I moved to a new place and tried setting it up as a primary router.

There is a modem that connects to coax, which gives Ethernet out.
My internet provider assigns three public IPs, so before I switched on the DHCP server, some of my devices got connected to the internet.

Set WAN to DHCP from Disabled
My...

RT-AC66U doesn't provide internet access to LAN devices, Shibby