@Chunmeng What do you have as your router now: brand, model name?

@phad Apologies, this is not expected outcome. With ISP Router it is supposed to work, and you did all tests for me to verify "the box" is router. If you didn't do that yet, power off both "the box"...

@phad Great. Then, what you have is router and it is configured as router. If it were modem or router configured in modem mode, only one LAN port could be active. Because it is in fact router, you...

@phad Thanks for the photo, very helpful. This box has four Ethernet ports. One is up, that must be one to which Main Deco is connected. Please do the test: connect laptop to one of remaining three...

phad wrote @Alexandre. It's connected directly into the modem @phad But it is modem, not router, right? Can you give me its brand name and model number, just to be sure? Some ISPs mistakenly name...

@phad OK, let's try some other things. Your Main Deco, is it connected to ISP gateway/router or to ISP modem?