so, I've had two routers connected via Ethernet on Lan ports for about two years with no problem. The first provided by my ISP is connected to the internet through DSL.
The second (ASUS dell ac68u) did all the WIFI heavy work connecting and managing all of my devices. So basically I use the router / modem provided by my ISP because it has VoIP ports to manage my phone lines.
Sadly this ISP provides internet and telephony services together and the modem / router is really bad on managing multiple devices connected to it. The problem occurred to me when I saw that the internet indicator on the ASUS was turned off, but I had internet connection. I tried connecting to the control panel of the ASUS but the ISP popped up.
I factory rested both of them but nothing changed. After that I tried changing the IP and the DHCP setting on both of them but still the problem continued.
When I manage them separately, without the Ethernet connection, I'm able to connect to their default control panel. It's like the ISP modem / router is using the ASUS modem / router only as an antenna but the WIFI SSD is that of ASUS.