Thanks for your invitation.
I write here to build and interface serial port.
To build openwrt firmware, you need to use linux OS. I use ubuntu mostly.
Openwrt is opensource and when you build, you can select router models and it will download all necessary packages automatically.
And then, upgrade to the router board, you can upgrade on luci interface or via serial console.
To connect to the serial console, you can use putty and you need usb-serial converter.
All routers have serial port inside, on the board itself.
Thanks.