I recently had to change my internet service. I had google nest with 3 wifi points connected to my old ISP. I did a factory reset on the google mesh once my new ISP was up and running. 2 of the 3 wifi points worked with no issues, but the 3rd wifi mesh point will not connect (QR code or manually).
What error do you get?
Sure it's properly factory reset and ready for setup?
Have you tried adding the 3rd point while it being connected by ethernet cable to the primary puck LAN port? Then remove ethernet cable afterwards.
Have you tried from a different device (Android, iOS or iPadOS) with the Google Home app, same Google account?