Are both rooms named “Office”?  Can you change the name of the lone 5 room then try to stereo pair the 5s?  Maybe remove the Sub first, pair the 5s then add the sub back into the stereo pair.

I tried it with different room names, removing the sub before trying the stereo pairs in different room names or the same. The app never sees the “other” play:5 and says that no identical speakers were found.. of course they have been previously paired for many years, so they are compatible. 
 

The concept of room used to stronger, more than a label. My theory is that for pairing you need to be in the same room, not just in a room that’s called the same. 
 

I had some strangeness when re-registered the “dead” play:5, it asked which room and “Office” showed up but with 0 speakers in it.. then on the 20th or so attenpt I saw an “office” room with one speaker and then it worked.. of course it should have shown 2: the second play:5 and sub mini. 

 Are all the speakers on the same firmware version?

 They should all be on 16.3.3.  I know that messes up compilation albums, but I think they all need to be on the same firmware version.

 Stupid question…. Are both 5s in the same physical room?  That Sonos term “Room” can confuse things.

 I know before I stereo paired my Move speakers they were listed under Your System as Move 1 & Move 2.  After pairing the listing under Your System is Sonos Move with Stereo Pair written under Sonos Move.

Did you create a second system after the speaker reset, rather than adding it to the existing system?

Hi,

after resetting both speakers multiple times, I connected to the WiFi network of both via my iPhone and that gave the option to connect to my home’s WiFi. Nothing worked straight away but after messing around for an hour, the option to create a stereo pair just appeared. The third time I did it, it actually worked. Yeah!

Now I can’t add my Sub Mini.. I reset it, and now it’s asking me to add it to a room.. but that fails every time. 

Stop resetting things. It rarely helps. 

Ok, I’m now at the stage where neither of them works at all anymore. They can’t link to “my account”.. so that’s probably a backend problem at Sonos then I guess?

A reset wipes any data that might have revealed the problem in a diagnostics report and a call to Sonos support. 
 

A shutdown of all your devices, your phone and your router might result in a stable environment when you restart everything. If you try this, leave everything powered off for a few minutes before first restarting your router. When it’s fully restarted and wifi is running restart your phone, then your speakers one at a time.