for some versions, I think the more dated one, there is a logout button

better make a comparison test between modem and router. make sure router has latest firmware

seems the router is faulty

did you reset the router? was you updating firmware or something that may cause this system error?

switch off and on can refresh the device list.

you may set the main unit as router mode, or connect all deco units through WiFi, just connect the main one to router/switch with cable, then for other units, you can set them in AP mode, link to the...