See the desktop version tutorial
Step 1: Login to your mobile app, then go to the “Discussions & documents” section of your team.
Step 2: Under the “Discussions” tab, click on the discussion topic for which you wish to post a reply.
Step 3: Write your review and click on “Reply” to send your message.
Step 4: Once it’s sent, your reply will appear below the other comments of the discussion.