https://kb.synology.com/en-global/DSM/tutorial/How_to_reset_my_Synology_NAS_7#t2

Here's how to reset a Synology NAS, just in case something goes wrong or you manage to break DSM.
How to reset Synology NAS and reinstall OS
If changing the administrator password is not possible or you simply wish to give the NAS a full reset, here’s how to go about doing so:

Press and hold down the Reset button for about 4 seconds until a beep is heard.
Within 10 seconds, press and hold down the Reset button again for 4 seconds until 3 more beeps are heard.
Wait for the status light on your Synology NAS to blink orange or green.
Enter the IP address of the NAS into your browser.
Follow the wizard to complete the re-installation process.
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