Curious connectivity issue - system only works with poor wifi?

Ok, here is a weird one, I’ll lay it out as simply as I can, moved into a new house:

• Router is at the front of the house. 
• 2 x Play 1 in a Stereo pair (Bedroom, upstairs)
• 1 x Playbar (Lounge, downstairs)
• 1 x Play 1 (Summer house, at the far end of the garden at the back of the house).
• All speakers connected via wifi, nothing wired in.

The bedroom pair and the lounge playbar are within the strong wifi signal zone.

The summer house speaker is riiiiiiight at the furthest point of the wifi, right where the signal is about to drop off and sits at ‘poor’. Sometimes it does drop off completely, thats how far away it is from the router.

Now, the system used to work fine but in the last couple of weeks (since I updated the app), performance has been tricky. Essentially the app can’t find the system at all - but only if I open the app in the house where the wifi is strongest. If I open the app in the summer house, ie right on the cusp of having no wifi, I have zero issues. The app finds the whole system fine, can search and play music across the whole system with no problems

Can anyone think why that would be? Why would the issues only manifest within the zone where the wifi is strongest? 

I’ve done all the usual resetting speakers, I’ve reset the router, reinstalled the app, wiped everything as if it was a brand new install blah blah and it worked for a bit, but then back to the same issue.

Next thing to try is hard wiring one of the speakers in to make a mesh, but I don’t want a speaker in the kitchen so that’s not a long term solution. A Boost would also be an option to try, but reluctant to spent money on more products to attempt to fix what should be working.

Thanks in advance!