Synology DSM has a default telnet password, and you can find generators online. It's unique each day per code that's part of the OS, so as long as you have telnet responding on the device, you can log in as root with the daily password.
Here's one such site, and there are many others.
https://wrgms.com/synologys-secret-telnet-password/
Sadly no Telnet active, anyone other ideas?