Thanks @Kevin_Z I haven't seen any error message on the Deco app other than to say it isn't connected to wireless. I have looked at the system logs through the Web interface (can supply copies if...

Home network has two Deco X20's with ethernet backhaul. Modem and master unit are in a hallway closet, ethernet over to a TP Link SG105E in the living room and the slave unit is connected to that...