I inform you that I solved the problem even if in a drastic way. I reset the phone (initialization) and when I restored whatsapp it finally asked me for my phone number and if I wanted to restore the old whatsapp backup on iCloud. So I was able to recover the lost message even if I lost all the conversations made in the period from the day of the backup to today. Not clear yet, why whatsapp does not allow to choose the back up in different (and more normal) way.
