I had the same problem, but somehow it was fixed after I sent error reports (from the Yeelight app) of the devices that would no longer connect. I tried lots of things before, but only after I sent the error reports the devices concerned suddenly turned online 5 mins later (and I had been busy with this for 2 hours before!). I would recommend anyone with this nasty problem to try it, it’s very easy and was the only thing that worked for me!
See my full post in the “conncetion timeout” thread: