Cannot update AW4 to watchOS 5.3.1
My new AW4 has watchOS 5.2.1, while the most recent available version is 5.3.1 (see https://support.apple.com/en-us/HT201222 ) . However I cannot update my watch since I use it with my iPhone 6 (iOS 12.4.2), and iPhone's Watch up says "to update the software on your Apple Watch, you first need to update the software on your iPhone". iPhone 6 can't be updated to iOS 13, I get it. So the AW can't be updated to watch OS6 - I get that too.
But why can it not be updated to 5.3.1???
Apple Watch