The thing that fixed it for me was: log every account off. Reboot everything. Connect to the same wifi. (I have 2.4GHz and 5GHz networks, being on the same network matters..). Log everything in using the same account. I'm ancient so my *apple account can be <me>@me.com, <me>@icloud.com and <me>@mac.com. Mixing those isn't helpful. Choose one (I suggest iCloud.com).
Once everything is synced, things should start to work. It's hasn't been useable in the past but the last few months have been pretty nice, bugwise.