At this very moment you are losing both new and current customers because of the app malfunctions caused by the servers that are down and cannot respond to new registrations and logins.
I Imagine this is fairly easy to fix and it should be prioritized provided that you have shut down the service as you said so no new attacks can be done.
You need to tell app users what is going on.
Add a sign up wait list for the registration function. If the regular servers are down tell the user trying to register that your services are currently down but you have taken note of the interest in your products and ask if they want to be contacted later.
if they answer yes then take note of relevant credentials like email address in a database on a small server. It does not need to be anything fancy it will just store the contact details for later.
Then if you ever get your services back up again and or able to take new registrations then you send a mail to all the email addresses in the database that they are welcome to register.
This is not a function you need to remove later. If your service go down again then this function will kick in again until you are ready again.
Show a status message on login failure if the servers are not responding to login requests. You could either do a simple message that your services are currently down but you are working on it or even try to fetch a custom status message maybe from the same small server as the one keeping the sign up wait list credentials.
Possible you should not even allow log in requests if the servers are down. Detect during the app loading if the servers are down and then replace the login function with a status message.