Had this issue with my new series 5 watch (apple care plus replacement for damage) and my iphone 11pro running ios 14.0.1. Issue was that my wifi at home is dual band and 5ghz is automatically used 99% of the time. After doing some network changes with the router and splitting the band temporarily to include 2.4ghz, everything downloaded, updated and functioned normally. It's 2020. You'd think Apple would have 5ghz support for the watch by now... Anyways thanks to this forum I was able to solve. Very grateful.