I don’t have any practical solutions for you I’m afraid, but I reached the conclusion a while back that my older kit, the Connect devices and other gear of that age had reached the limitations of their hardware.   The Play:3 is about 12-14yrs old, and I don’t know anyone who is using a mobile phone or a computer of that time period.  As a software driven device its life expectancy must be considered in that context, as it is not like an old vintage amp or vintage speaker that you might expect to go on for many decades.  Sorry, that probably is not what you want to hear.  

Try another mobile phone to do this. I had the same issue with my Sonos One SL, I used my wife’s phone then it registered. But I still have issues with grouping the speakers

Just to follow up - I tried support again - this time the estimated wait time was 80 minutes BUT the automated system gave me an option to get a call back. So two hours later i got a call back and after an hour on the ‘phone 6 diagnostics and transferring ownership from me to me the Play 3 is fully S2 and connected !