As a co-author of a manuscript submitted to Frontiers, you will be notified for all actions regarding the manuscript. Moreover, you can directly see and reply to reviewers' comments.
If you are not registered, you should register at Frontiers with the email used for the submission and the article will automatically be added to your account.
If you already have an account on Frontiers, then it is likely that the address used in the article is not validated in your account. Please add it and validate it so that the system can make the link.