@pdeseymour 

I have one of my Satellite Deco connecting through two switches. It is supported configuration, when everything is right. You can find in just four steps that I will list below what is wrong.

Proceed to the next step only after success of the previous step. For tests, use M4 you plan to place to garage.

1. Unplug switch directly connected to Main Deco from the Ethernet cable. Connect M4 to that end of Ethernet cable. Does M4 run on Ethernet backhaul?

2. Plug switch back and connect M4 to spare port of it with short Ethernet cable. Does M4 run on Ethernet backhaul?

3. Plug long (50m) Ethernet cable to the spare port of the switch, take M4 to garage and connect M4 to the other end of that cable. Does M4 run on Ethernet backhaul?

4. Connect unmanaged switch to that long cable, in the garage, and connect M4 to the switch. Does M4 run on Ethernet backhaul?

  @Alexandre. Thanks for the response.  All the checks in your response have passed.  To be more specific about my concern, as I do not have WiFi in the garage, would the Deco work on a single Ethernet connection.  In the back of my mind, I thought the backhaul needed WiFi.  If this is not the case, and the M4 can work on a single Ethernet, then the only point of failure is the switch in the garage which is working ok with CCTV.  (and sorry about the typo in the Subject!!). Cheers

  @pdeseymour 

M4 can work on single Ethernet connection, it does not require WiFi connection to the rest of the mesh. 

Step #3 from my list checks that long Ethernet cable to the garage is OK for M4, and step #4 checks the switch in the garage.

Not every switch is compatible with Deco Ethernet backhaul. If the one you have does not (Step #4 failed) and you plan to get new switch, TP-Link brand strongly recommended. TP-Link 5- or 8-port unmanaged gigabit switch the most likely to support Deco mesh Ethernet backhaul.

I also have unmanaged switch from TRENDnet, it is compatible with Deco mesh, too.

If Step #3 is successful but Step #4 is not, you might not need new switch for garage. Keep M4 connected to that long cable, and connect existent switch to second port of M4, and to that switch connect CCTV camera. 

  @Alexandre. Excellent.  Many thanks.

Alexandre, just an additional piece of info, the switch I have been trying was bought yesterday and is a TP-Link 5 port unmanaged!  I'll try the Trendnet.  Rgds

  @Alexandre. Hi , I started again today with a new Trendnet switch.  I connected the switch to the switch connected to the Master M5 and used the port that sends Ethernet to the garage.  I connected the M4 to the new switch.  No connection established.  So I connected the M4 directly to the port on the switch (connected to the master) that goes to the garage.  I got a data light on the switch but still no connection, just the blue flashing light. I know this M4 works on WiFi.  So, I took one of my M5s and connected it in the garage directly to the cable from the house.  Got a full, stable connection immediately.  Strangely, this is now pointing to a faulty M4.  That's easy to prove now as Ill just place it where the M5 was.

Rgds