I contacted the Theme Developer and he tells me that the theme is using standard WP pagination functions and I have confirmed this by activating various other themes including the default themes and have the exact same issue.
Significantly, by inserting category
into the URL it all works fine, I get to see the second, third etc pages.
So, why do I not have ‘category’ in the URL? i.e. https://mywebsite.co.nz/category/my-category/page/2/