Cesar

Wow! That workaround worked! It's really weird and annoying that you cannot start a router the first time with the ethernet cables plugged to another router/modem,

So, the workaround that also worked for me is: "unplug all cables connected to the router, shutdown the router by pressing the button and once all cables are unplugged turn on the router, wait the 4 minutes, and plugin all cables, you should now be able to get into the interface".

Link to the issue I faced with more details, solved with the workaround mentioned by @liluziflirtttt Thanks man!
 

the only workaround that I know is to reboot the router by pressing the button and unplugging all cables connected to the router, Once all cables are unplugged turn on the router wait the 4 minutes, and plugin all cables, you should now be able to get into the interface