Raspberry Pi4 as NAT Router

Sorry for my bad english
I have compiled an OpenWRT image for my pi 4, I want to connect the pi to my ISP modem (layer 2), get a public ip from the ISP cloning the MAC Address (the isp has no problem with that) all of that using the integrated LAN port on the pi, then I want to use the pi as Router/Gateway for my whole network, I'm connecting a dumb switch to the pi via a USB 3.0 to ethernet conector (i have the drivers and it works perfectly fine).
I don't know how to do the packet forwarding on OpenWRT, there is no WAN interface when I installed OpenWrt only one LAN interface
Please help!!!!

just go to interfaces and add a wan interface and assign it to the wan firewall section, then add the interface where you want the WAN to the physical settings tab

I did it, the pi is getting internet trough the WAN interface (ping works) but is not giving internet to the devices connected on the LAN interface

onboard is more resilient for LAN as always present on boot. reset / re-install and use the 3.0 for WAN.

Sorry, I didn't see the message before, I have switched the interfaces, but my problem is that I can't make the LAN interface get the internet from the WAN interface

uci show network | grep ifname; uci show firewall | grep -E '(forwarding|masq|network)'