Tafsir of Nuh 71:9

ثم إني أعلنت لهم

Then verily, I proclaimed to them in public, meaning, with open speech and a raised voice.

وأسررت لهم إسرارا

and I have appealed to them in private. meaning, in discussions with them. So he tried various types of propagation to be more effective with them. What Nuh said when He called His People to Allah

فقلت استغفروا ربكم إنه كان غفارا