Refresh token not working

Currently I am trying to migrate our application to the new refresh token system

I am getting my Refresh token and my ExpiresAt values, and saving that data on the database,

if the ExpiresAt value has already pass I am creating my object this way:

 dbx = new DropboxClient(settings.RefreshToken, appKey, appSecret);

and using that new object for my resquest, but in my view I am still getting the message:

Failed to access a remote cloud service
StatusCode: Unauthorized
Request result: {"error_summary": "expired_access_token/.", "error": {".tag": "expired_access_token"}}

where in c# I have to sent my refresh token value so it can keep the token alive when I need it?