@R-ehv 

This feature is not available in some Deco models. For M4, it is even more complex: some M4 variants have that feature and some don't. 

Usually, this feature availability can be established from Deco firmware version. If you can make screenshot of Update Deco page in Deco app and share it here, it'll help to figure what you have. Example of that page:

  @R-ehv 

This variation of Deco M4 does not support Connection Preference feature with its current version of firmware. It appears you purchased the whole set from one place, all three units are the same.

Here are options you have to get this feature:

1. Install M4 beta firmware which supports that feature, it must be installed successfully on all three units to work. See this forum topic: https://community.tp-link.com/en/home/forum/topic/651248

or

2. Return this set to retailer and get different Deco model, the one which has all its variations support Connection Preference feature. If similar to M4, these would be M5 and M9 Plus. If newer model, the one that supports WiFi6, these would be X50 and X55.