Network (wifi) Selection Question

I have a router broadcasting wireless network SSID (A) and a network extender broadcasting SSID (B).

I want my phone to connect to the extended network (B).

I have a smart app running that when it recognizes my home location it turns on wifi and turns off data.

I go into my wifi setting and if I see that I'm connected to (A) I change to connect to (B) and that works fine.

Now, sometime later I check my wifi connection and I'm connected to (A) again.

How do I set the phone to select (B) as the preferred network to connect to, similar to Windows when you select a network to connect to automatically?