@RandomStranger 

See the following FAQ document: How to extend guest network when there is a switch between Decos

  @Alexandre. Problem is i don't have a managed switch between the M9s, i am case 1 - which should "just work" but isn't.

So i don't even get the option to set the vnet on the satellites when i try to load them via the web UI. So never see the L2 FEATURES>>VLAN>>802.1Q VLAN>> options.

Not sure how to proceed.

  @RandomStranger 

I do have Deco mesh with M9 Plus, Satellite Decos hardwired through unmanaged switches. I have number of devices connected to Guest Network on Satellite Decos, so I know that at least in some cases it "just works."

If you would tell brand and model of your switch, that might help TP-Link Support reading this forum. 

Still, if this is truly unmanaged switch you do not have control over, not much can be done other than replacing it with the different switch. Just to test it is the switch, eliminate it from the connection chain to Satellite Deco and see if when Satellite Deco is wired directly to Main Deco it'll accept connections on Guest Network. For that test, you can just unplug the switch from Ethernet cable going to Main Deco and plug Satellite Deco to that cable. If you don't want to disconnect wired devices from Internet for too long, connect second port of Satellite Deco you use for tests to the switch.

  @Alexandre. Yeah, thats a good idea.

I just tested by unplugging a Satellite Deco, then connecting to it - and it worked.

So it could be the switch. The switch is a Netgear G5205 https://www.netgear.com/support/product/gs205 - so your idea of connecting directly to the main Deco is one i will try tonight. Thanks!