Can not set "Split post by:"

Great plugin,

After Navigate to Settings > Reading in WP Admin

But Unfortunately, I Can not set “Split post by:”

Either “Total number of pages:” or “Approximate words per page:” does not work.

After do setting “Total number of pages:” other than 2, then back to default 2

Also, After do setting “Approximate words per page:”, again, then back to “Total number of pages:” to default 2

Please help me,

https://www.ads-software.com/plugins/automatically-paginate-posts/