Some high-end Synology units feature a separate Ethernet port, labelled MGMT which is the Out-of-band Management port.
Once this port has been assigned a static IP address (you can not enable OoB Management on a DHCP address) then you can turn on Out-of-band Management.
Go to Control Panel > Network > LAN Management (Out-of-band) > Edit.
Assign the interface a manually configured IP address, and then you can go to the Out-of-band Management tab and enable OoB.
Tick the checkbox to turn it on, and then check the port number – the default port is 57, and you can not change this to port 22 as sshd is already using port 22.
Once this has been enabled, you can ssh to that interface, on port 57 – however there is a catch. Synology appear to be using really old key exchange algorithms and ciphers, so you may need to tell OpenSSH on your computer to enable them for this connection:
ssh -oKexAlgorithms=+diffie-hellman-group1-sha1 -oCiphers=+aes128-cbc [email protected] -p 57