As per [this](https://support.google.com/webmasters/answer/75712?hl=en) page sitemaps can't include other sitemaps from different domains where the other domain hasn't been verified by google webmaster tools. Instead use a html sitemap populated by js, as per [this](https://www.youtube.com/watch?v=t5LIlkhxl2s) vidoe google's bot should be able to go through this and thereby index all pages.
