I am at a loss, only home page is indexing (URL in thread)

Let me start by saying I am not an idiot, I've been doing SEO for almost 20 years, however with a new website we've acquired, I am either missing something really basic or there is something else going on. The site in question is OutdoorPhotographer.com, I've checked all the basics like the robots.txt, WP own settings, Yoast SEO settings e.t.c. The two things I've thought it might be is: 1. Thin content, a lot of pages are user submitted photography pages which I've ensured are no index, alongside various other thin content pages. 2. Content lock, so a lot of the content is behind a membership wall, you can view 3 pieces of content and then have to sign up. The thing here is, we have this on other sites and it's fine... Anyone see something obvious I am not seeing :/