Thank you for the advice Chelsea_Ring. I had already partly implemented what you suggest but, for reasons that will become obvious, it could never really be a full solution. The problem devices are 2 StickUp Cams in an outdoor aviary, keeping an eye on an elderly tawny owl. The aviary is at the far end of a chain of wifi mesh devices, where I intended the cams to connect to Mesh Disc 5. Mesh Disc 5 is connected via Disc 3 or 4 depending on how the mesh configures itself. Unfortunately the cams had a habit of connecting to the weak signal from Discs 3 or 4, instead of the strong signal from Disc 5. I had switched off Disc 3 without the mesh collapsing, and that did partly alleviate the problem, but the cams still did sometimes connect to Disc 4. I am sure you can see where this is going now - if I turned off Disc 4 as well as Disc 3, then Disc 5 no longer had anything to connect to, and neither did the StickUp cams!
I have now had to do what I wanted to avoid, and run an ethernet cable out to the aviary. It had to run up through a ceiling, through a loft, out through the eaves, and overhead to the aviary. It then connects to a repurposed router reconfigured as a wired access point with a different SSID. The cameras have been permanently connected to this SSID, and are off the mesh system. I was hoping that the wifi mesh system would have saved me all that trouble, as I am far too old to be crawling around in lofts, but a proper wire is always the best solution.
I had to implement a similar ethernet run to the other end of the house when I found that power-line networking caused terrible interference on my guitar amplifiers. Sometimes the magic of clever tech works, and sometimes it doesn’t. Thanks again for your help.