I was having a horrible time trying to Factory Reset my Sonos Beam and I found this forum with a similar question - and no answer.  I keep looking and I found one person who fixed theres - they had to hold the reset button for OVER A MINUTE !!!… So in frustration I did the same thing - I unplugged my Sonos Beam from everything - waited about 2 minutes - Plugged it back in holding the “Infinity Button” on the back and at about 45 seconds it started blinking amber and white.  I simply had been holding it for like 15-20 seconds and giving up… but this worked and my Beam reset and is on my new wifi attached to a new system.  I hope this helps someone else from loosing an expensive piece of equipment