I’ll not be bothering with another router as further investigations have eliminated this as a cause to my mind. I am sure that everyone here that has suggested routers as being the issue has been doing so in good faith but I am confident that is not the issue, at least for me. I’ll outline how I reached this conclusion below for the community.
My smart device problem has been getting worse of late so that, even though the issues have been intermittent and inconsistent, I had reached the point where there would always be at least one issue somewhere. My example is probably more extreme than most because I have over 50 devices, 12 of which are Yeelights. Always having some problem or other has at least now enabled me to troubleshoot further though.
The issues always appeared to be with Yeelight products showing as offline in the app. Other devices from other manufacturers were starting to be affected in that they would sometimes miss IFTTT events but would always be there in their respective apps. This wrongly led me to suspect general issues with my network and so I had been trying different network components over the last few weeks yet the problem persisted.
Yesterday, I thought I’d reserve addresses so that all my Yeelight products were allocated addresses next to each other. To my surprise, I found that a number of the Yeelight devices were using IP addresses that were also claimed by other manufacturers’ devices. This was a Eureka moment because all of the symptoms I had experienced could have been caused by duplicate IPs. I continued to reserve addresses and found that some Yeelight devices would not use the new range even though the leases must have expired on the addresses they were using. Restarting everything (including the the router with DHCP enabled, my wireless access points and the wifi extenders to force the issue) did not make the difference it should have for these suspect Yeelights although it did work for other manufacturer’s devices and some Yeelights.
Next, I disabled DHCP on my Belkin router and replaced with a TP-Link router. I left all Yeelights off while setting this up and configured the TP-Link router with the DHCP reservations. I then restarted the Yeelights one by one and they were all correctly allocated my chosen IP addresses for each of them. All looked good in the app and I was feeling unduly smug. Everything from all manufacturers started to work perfectly and consistently. I at last had regained a working Smart Home.
Everything worked perfectly until this morning that is. I now find the same 4 suspect Yeelight devices are intermittently showing as offline in the app. Everything else by every other manufacturer is working.
I have confirmed that the suspect devices are definitely retaining the correct IP address and I still have no duplicate IP addresses which is something. Pinging the suspect devices though shows intermittent dropped packets and responses from 2ms to 400ms. The Wifi strength is definitely good and there is no channel conflict with neighbours (confirmed using various apps on my mobile but also evident with the Xiaomi Wifi Range Extenders currently placed next to the Yeelights remaining connected throughout and reporting as healthy in the Yeelight app).
I have now found that switching off the suspect Yeelights for an extended period (say 30mins) makes them reliable at first but leaving them on gradually degrades their Wifi reliability. I suspect this may have been the reason the DHCP was going awry for me before (with them requesting an address but not successfully responding with an ack or maybe not successfully renewing fully). This may also explain why some other users have found changing routers has helped (maybe the new router’s DHCP is more tolerant or maybe the act of the changeover and forced reset fixes things for them in the same way it did for me very temporarily).
Regardless of the DHCP challenges, my Yeelight failures clearly cannot be resolved by changing my router. I can say this with absolute confidence because I can test the local device connectivity with pings even with my router turned off. The suspect devices and my laptop are both connected to the same Xiaomi Wifi Extender and both in the same local subnet so that there is no router involved in the ping and I still see a gradually degrading success rate. I have tried different wireless access points for the extenders just to eliminate that as a possibility and this makes no difference.
I am in the situation where I have some Yeelights that are rock solid and others (exactly the same model) are unreliable even though they are all connected to the same environment on the same account. With them all running the same firmware, my only conclusion is that issue is actually a hardware fault and I will be removing the suspect devices but keeping my other Yeelights. If this is the same for others that have been reporting similar unreliability then it may be a general quality-control issue with the hardware. It may be others have other issues though of course - this has just been my experience.