I have my ISP router (192.168.100.0/24) in the first floor of my house and where all ethernet cables are going to from each room.
Then I have my main TP-Link Archer AX6000 router (192.168.0.0/24) in the second floor with WAN port connected to the ISP router in the first floor.
I do have some devices across the house that uses ethernet but they only can connect to the ISP router in the first floor so they get 192.168.100.* IP.
I want to connect the ethernet cable of these devices to the ISP router and then connect it back to the TP-Link router in the second floor so they can have the same subnet of the TP-Link router (192.168.0.*) and manage the device from there.