I’ve been struggling to update after recently downloading & installing the Live ISO image.
This is how I got it to work. It complained about only one download failure for some delta patch then continued on to completion.
sudo swupd update --max-parallel-downloads=1 --ignore-time --nosigcheck
Finishing with Update successful - System updated from version 37290 to version 37480.
CURL was complaining about retry timeouts, partial downloads & some 504 error. When I installed Clear Linux I got lazy & left the timezone to UTC, then set it correctly later. I don’t know if time being wrong was a problem.