I want to ask you something, my line is fiber, I do all the settings, but I couldn't go to the internet, I need to enter the mac address of the other router, might it be that gpon doesn't recognize it?
Some internet providers don't like certain MAC addresses from some routers or sometimes they allow internet access to the first router that they register.
You did fine cloning the MAC of the other router or you can use the MAC of your computer's network card. Or you can try to reboot the ONT (fiber modem) to see if it accepts your Xiaomi Router's MAC.
I haven't linked yet, I need to write my mac address