I had this problem and went around and around with a bunch of suggested fixes as above, and then also found a lot of reference to 'Advanced Networking' which enables apps to have 'local network access' which ultimately means 'internet access'.
The issue is that the 'Advanced Networking' feature has been moved and renamed, so if you look for it in the WSA settings, you won't find it.
The setting is now called 'Local network access' and is located on the 'Advanced settings' page, under the 'Experimental features' section.
'Experimental features' section first has to be expanded (1) and then you can toggle 'Local network access' (2):
For good measure, I then turned off the Subsystem from the System page, and the re-opened the Amzon Store app which automatically starts the subsystem back up.
When I clicked the login button - I was immediately presented with the Amazon login page.