I have two of the Sonoff S31 Lite zigbee plugs. I will not buy any more.
I bought them because I needed a repeater for a color zigbee bulb, I found documented here that they work as repeaters, and they were very low cost (about $7.50 each) on an Amazon sale.
First two shipments, Amazon sent me WiFi plugs even though I ordered the zigbee.
When I finally got the zigbee plugs, they paired ok (I had some trouble with one of them).
Plug A worked fine until last week. Plug B usually worked (95%?).
Last week they both stopped working. I could not figure it out. Per advice of the very patient people on this forum, I tried to re-pair them.
Plug B re-paired right away.
Plug A would not re-pair, would not respond in HE, and would not factory reset. So I moved things around. Put Plug B in the critical location plug A used to be. It works, and repeats for the zigbee bulb.
Put another zwave device where Plug B used to be to cover that function.
Put non communicating Plug A in a new location, further away from the hub than either of them ever were. I figured it was just a plug with a button. 2 days later it connects to the HE on its own and starts working.