I've installed the beta firmware for both M9+ and M5. It works well for me with correct source now. My config is 2 units of M9+ & M5 each for a 3 story house. The speed from weakest spot has improved by 2-3x with correct source. 

Do contact support to install and try out the beta firmware.

@YCNg where did you download the beta firmware?

@PatrickFaria Private message @TP-Link and let them know you want to test the beta firmware and the model M5 etc. If there's mix of models in your network, you need to upgrade all firmwares as well.

@YCNg 

Could you please briefly describe how the option is implemented in the beta firmware. You cannot select which deco connects with the one chosen by the user. I have 5 decos (I just added one) and the four decos satellites are irretrievably connected to the main decode which is connected to the ISP. Thanks and best regards

@Rioja Yes, there's still no option to select the source. The updated firmware is to optimize through "Network Optimization" feature.

I've a mix of Deco M5 & M9+, 2 units each but the two units having wrong source issue were Deco M5 located at Level 3 of my house. After updating the beta firmware for both models, I clicked "Network Optimization" from the app and the two units turn out with the correct source. 

Since it's in beta stage, it may not work well for every layout but worth to give it a try.

@YCNg Hi there, like your case, I have a mix of 2 x M9 and 1 x M5, and annoyingly the M5 keeps connecting to the farthest M9 which is the main deco.

Is the beta firmware (M5 & M9) downloadable by public and can their URL be shared to all? Or anyone who would to try them has to PM the TP Link support individually in order to get the beta firmware files?