SSH login is usually handled by the the server provider. Cloudron does not have a UI to manage SSH login methods. If you are locked out of your server, usually your VPS provider has a way to reboot the server with a SSH key setup or a reset root password.
Otherwise if for some reason this does not help in your situation you can try to enable Cloudron remote support in the support view of your Cloudron dashboard and then send a mail to email@example.com with your domain details. This may allow us to reset your SSH login details.
To add here, SSHd configs are very often VPS provider specific even, not just Ubuntu. So ideally Cloudron should not try to manage too much around that, since then this might interfere with for example SSH recovery strategies from VPS provider.
Generally it is always a good idea to use ssh keys instead of password.
Currently there is no supported way to connect to the MongoDB addon instance from the outside. Apps can only access it via the local network on the server itself. Can you describe you use-case a bit, so we can see if we should add such support? Thanks.