I already have purchased the 2 M3W devices
As described, *very* clearly here: https://www.tp-link.com/uk/home-networking/deco/deco-m3w/
> Deco M3W is compatible with all Deco routers to form Mesh network. Enjoy all functions of the main Deco router* wherever you are at home.
>
> Check how to configure mixed Deco network >>
>
> *The functions of mixed Deco system are consistent with that of the main Deco router.
Note the use of the * to emphasise that the *main* Deco determines function:
> *The functions of mixed Deco system are consistent with that of the main Deco router.
This clearly isn't the case!
The presence of the M3Ws *prevents* selection of which WiFi A/P to connect to - one of the *best* features of 1.6.x firmware