Ne ... plus = No more, no longer (negation)

Bonjour Jennifer,
While Aurélie does an excellent job in providing the pronunciation rule, I have located a couple of other sites that, perhaps, can give you some added insight:
https://frenchmoments.eu/pronunciation-of-french-adverb-plus-when-to-sound-the-s/
https://www.youtube.com/watch?v=fvc6KpW7jfw
https://www.youtube.com/watch?v=Uv7nTsxUy30
https://www.youtube.com/watch?v=UaA8pcL8LeQ
https://www.thoughtco.com/learn-french-expressions-with-plus-4084866
I, too, have difficulty in remembering exactly when to pronounce the «-s».
I hope you find these helpful.
J'espère que ma réponse vous aiderait.
Bonne chance et bonne continuation dans vos études en français, la langue de Molière et qui a été utilisé par le monde français depuis l’époque d’Hugues Capet

The grammatical answer is: when "plus" is used as a pronoun meaning "more" you pronounce the final s. When it means "less" or when it is used as part of a negation you don't pronounce the s (unless it is followed by a vowel).

Plus de messages (with silent s) = no more messages
Plus de messages (with s pronounced) = more messages

In this case you can't devine the meaning unless you know the context or you hear it spoken.

-- Chris (not a native speaker).

Thank you both for that. I will be checking out the sites you quoted Ron and Chris your answer reminds me why I find it so difficult to remember when to pronounce and when not.