A PLAY:5, depending on which generation it is, can be fairly old. Have you called Sonos Support directly to discuss it? It’s possible the network card may have failed. Or, there could be something else wrong…

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.

I connected the malfunctioning Play 5 to my router and it initially worked but dropped the entire rest of my system. Out of frustration I abandoned the project and came back to it two days later. At this time the whole system is recognized and working properly. The Play 5 is still connected to my router and the whole system is running on Sonos channel 6. I had in the past factory reset the problem Play 5. I can only surmise that the software updated and now it is the speaker that all of the others connect to. Thanks so much for all of your advice. I hope it lasts. 

I recommend that you refrain from Factory Reset without further consult. A Factory Reset rarely cures issues permanently, although there can be accidental temporary improvements. Plus, the Factory Reset creates extra work and destroys diagnostic data that could have been used to discover the root cause.

I suggest that you pick through your router documentation and learn how to “reserve” IP addresses. (Sometimes this might be described as “fixed” or “static” IP addresses.) This is a boring chore that does not need to be repeated until you replace the router or add more Sonos units. (But by then you’ll be much smarter.)

Community regulars have done this years ago and we have not been dragged through the frustration that you are experiencing (and are likely to experience again) since we made our reservations.