That's good news. Glad it worked out and that you didn't have to buy any additional hardware.
I believe the issue regarding your original folder arrangement is more an issue with DS Video then Plex. DS Video set that folder aside for its own special purposes and it is more likely disallowing...
I ended up starting with a clean install so I could move up to DSM 7.01. Plex is an App you need to download to install. I recall I had to download a Beta version of Plex to work with DSM 7.01. I downloaded it to my workstation and then used the Manual Install approach from the Package Center...
I've had a Synology NAS 215J since 2015. Up until 5 months ago I used it for playing back MP4 videos on iPads and Samsung TV's. The files were compressed for general mobile use, and I never had a problem with lag. However, in the last 5 months I've switched out my wireless router with an eero...