Ordering relatedTo entries
First you’ll need to get the Table Of Contents entry that holds the Entries field (the source entry). I’m going to assume that is in a Single section with the handle toc.

{% set toc = craft.entries.section('toc').first() %}


Once you have that, you can just pass its Entries field value into getPrev() and getNext():

{% set prevEntry = entry.getPrev(toc.myEntriesField) %}
{% set nextEntry = entry.getNext(toc.myEntriesField) %}