I have three M4's in my house.

One is around 15m from the back of the garden, the other is around 25-28m from the back of the garden but elevated.

According to the app, I am connecting to the one that is further away, which causes a drop is signal.

The closer one is behind a wall but when forced, it'll connect to it.

Is there anything I need do change specifically to fix this?

I need to test better to see what's going on as I wonder if the Deco that is downstairs is connecting to the upstairs one (it's probably physically closer), which then connects back to the main M4 in the front of the house.