The Florida Rental course is a temporary course, the certificate only lasts for 90 days.
The only way to receive a new certificate after the previous one has expired is by retaking the course. This however requires you to create a new account under a different username than the one you originally used.
Please consider taking the lifetime course at https://www.boat-ed.com/florida/ if you would like a permanent license. The course may take longer to get through however, afterwards you would have the license for life.
The rental course can be accessed at www.boat-ed.com/floridarental.