If you haven’t disabled SMBv1 everywhere, on every PC, NAS, server, you should.
https://www.synology.com/en-uk/knowledgebase/DSM/help/DSM/AdminCenter/file_winmacnfs_win
After disabling SMBv1 on a Synology NAS with DSM 6.1.5-15254 Update 1, I could no longer mount the shares from Linux. On Linux Mint 18.3 with KDE, you can’t select SMBv2 or 3 in the Dolphin or Smb4K GUI (yet) so you have to mount it from the cli.
Both smbclient and mount worked when I specified the SMB version. Note that both commands will prompt you for your password.
user1@lmint ~ $ sudo mount -t cifs //192.168.1.10/data /home/user1/Synology -o username=user1,vers=2.0,sec=ntlmv2
user1@lmint ~ $ smbclient ‘//192.168.1.10/data’ -m SMB2
I think there’s some missing info here: you have to make sure the maximum SMB level is set to 2 or 3 using File Services/SMB/Advanced