Paste the file from the microSD card. Checks run in this browser and nothing is uploaded.
A serial number fills the MAC addresses from the decoding table:
eth0 is 9A:00:00 plus the first three octets of the serial, eth1 is 9A:00:01 plus the same three.
The controller only applies eth0 and eth1. When both have a gateway, the one later in the file replaces the default route.