Follow the Synology How-To:
https://kb.synology.com/en-global/DSM/tutorial/How_can_I_recover_data_from_my_DiskStation_using_a_PC
Tip
For mounting the encrypted folder use
ro and
mode=0777 option
mount -t ecryptfs /mnt/tmp/@EncryptedFolderName@ /mnt/encrypted -o ro,mode=777