I have 3 NAS, 2 of which are used to store my flac files. Roon is running on a DS718 NAS (SSD). It is impossible to configure the shared folders from Roon. I get an unexpected error :-). My Mac, my PC have no problem to mount these share points. There is no special character in my password. The only solution was to mount the folders via Synology’s feature of mounting remote share points from the Roon NAS. I created a Mount folder on the Roon NAS and mounted the share points of the other 2 NAS through it. This is a workaround because my folders are scanned across all the share points (it’s long, very long).
This is a restriction of DSM 7 AFAIK. You already found the solution to circumvent this limitation for apps on Synology’s OS.
As providing storage space is a main purpose of a NAS, you could also copy the contents from the other NASes to the one running your Roon Server so all the music is available locally (preferred way of operating Roon Server). You can still keep the other locations (NASes) as backups and/or for other purpose (other music server software).
I assume that you are running DSM7 on your DiskStation. Starting with DSM7 (and onward) applications (packages) are no longer allowed to make a external (SMB) connection. You (as you already discovered) first have to make a external connection in File Station and then within Roon you can select this created connection.