This thread is resolved. Here is a description of the problem and solution.
Problem: How to translate "next page" label?
Solution: Go to WPML String Translation and try to locate the string using the text search field. Enter the literal complete string and enable "Exact search".
In case you are not able to locate the string please scroll to the bottom and select "translate text in admin screens". I would advise you to check here any domain related to the currently used theme. Try to locate the string. Once located mark the checkbox for the string, scroll to the bottom and apply your selection.
The string is now available on String Translation.
Relevant Documentation: https://wpml.org/documentation/getting-started-guide/string-translation/
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