iphone wifi hotspot selection

Hello!

I have two hotspots for my home network - one in the house and one in the garden office.

Recently I found that my phone is preferring to connect to the garden hotspot when returning home and this was causing connectivity issues with my homepod (connected to the house hotspot).

So I turned off auto-join for the garden hotspot. The intention was that I would manually select the garden hotspot when working during the day, but for the rest (majority) of the time my phone would prefer the house hotspot.

However I am finding during the working day that, despite manually selecting the garden hotspot, the phone, particularly after the screen locks, is reverting back to the house hotspot, despite the signal being so weak (can't even download whatsapp messages).

Any idea why it would do this? In the garden office the house hotspot signal is clearly weaker (-80db v -45db) but, with auto-join enabled, it seems to rank the weaker house hotspot higher. What's more, once manually put on the stronger garden hotspot, the iphone does not move out of range, which is usually when a device searches for a better hotspot.

Thanks in advance,

Matt