Interesting: a PC would not find the upgrade. Signed in with an MS Account, Fast Ring selected. Refreshed, restarted the WU service, nothing. Some Defender update, no new build.
Just to test all possible scenarios I created a new user account, with another MS / Insider account of mine. Made the new account admin, signed out from the old one and signed in to new one. Removed the original MS Account, rebooted and signed in with the new user account with this other MS / Insider account of mine and it immediately started to download the 10166.