@David-TP 

  @jeff423 Hi, welcome to the community.

Is there any network switch between the Deco units?

Would satellite units turn flashing red during the disconnection?

Which of the following is quite like your current network topology?

Verizon 5G wireless modem router---main Deco X60---Switch--satellite Deco X60#1

                                                                                                \---satellite Deco X60#2

                                                                      /---main Deco X60

Verizon 5G wireless modem router-----Switch--satellite Deco X60#1

                                                                      \---satellite Deco X60#2

Or

Verizon 5G wireless modem router---main Deco X60---Switch--satellite Deco X60#1---satellite Deco X60#2

My configuration is:

Verizon 5G wireless modem router---main Deco X60---Switch--Switch---satellite Deco X60#1---satellite Deco X60#2 (I have two switches because I need to connect my desktop computer with a cable)

Both satellite units are connected directly to the second switch with cat 8 cables.  The first switch is a TP Link TL-SG105 and the second is an old Netgear 10/100 unit which I'm currently replacing with another SG105.

The lights were green but the Deco app said they were both disconnected.  When I disconnected the ethernet cables both went back to working but were slower that when connected via cable.