@Mustaqim 

As of today, preferred connection is available for the following Deco models:

Deco M5
Deco M9 Plus
Deco M4 (hardware V4)
Deco X50 (hardware V1)
Deco X55 (hardware V1)

Consider returning Deco X68s and replacing them with Deco X50/X55.

Just note that X68 is tri-band, while X50/X55 is not, which means you might see slower WiFi speeds on Satellite X50/X55 comparing with X68.

For Deco M4, if you happened to have them on hardware V4, you can update them with beta firmware that has preferred connection option, from here: https://community.tp-link.com/en/home/forum/topic/581554

Will the preference option be available in future software update?

  @Mustaqim 

I advise anyone before buying a TP link Deco product is to look at the products support page , the one that lists the firmware currently available and check if new features are being added ( connection preference- basic feature to be honest)  etc or even the last time the device received an update. 

Even more confusing is the fact that you can have the same product sold under the same name but can have different hardware versions 1.0, 2.0 which potentially has different features. 

TP link are very very slow at firmware updates on still very current products.