Based on both of the points you said, I believe The following:
For the second point only:
Many NAS Devices with built in BitTorrent Clients work, but, are not very good. It is possible that it does not support DHT and the tracker you are connecting to is either offline or the file does not have any peers.
Your NAS either does not support UPnP, or you have UPnP disabled on your router, or you have not set up your firewall ports correctly.
The trackers you are connecting to are private and/or have some kind of status that they are returning but your client is not showing the message (e.g. wait time).
However when I also consider the first point combined with the second:
I believe that your NAS does not have an active internet connection. If you do not have DHCP active on it, make sure it's gateway and DNS are set correctly.
This would explain why it can not download/access .Torrent files via URL but you can put them there from your LAN, however, they do not progress at all.
I think I have covered all the possible reasons, I believe the last paragraph is the real reason, but, unless your NAS has a command prompt or any way to ping a site, I am not sure how you can test.
All this being said, if you turn your machine(s) off at night and just want the NAS to be a low powered Torrent client, fair enough - however, I have to say that a full torrent client such as Utorrent will give you a much better experience.
Best Answer
In μTorrent you can right click on any torrent >
Bandwidth Allocation.High/Normal/Lowindicates relative priority.It's also possible to set torrent-specific upload speed limits through this menu.
When a torrent is set to high priority, (almost) all bandwidth is taken from other torrents and given to it until it's satisfied, i.e. the people at the other end are downloading at their maximum speeds. When there are no other peers for high priority torrents, other torrents behave normally.
This setting applies to both downloading and seeding torrents.
There's some minor discussion on it here.
It's also possible to set priorities on individual files, but I'm not sure if that applies to seeding.