How to get rid of sticky cough and phelgm for my 11mths old.

My baby had covid when he was 3mths old. Not sure if it's due to covid that cause him to take longer time to recover from cough/ flu/ fever? Any mummies or daddies have natural remedies to recommend? We tried various medicine and it doesn't seem to get rid of it completely ##pleasehelp #advicepls #help #sickbaby