it's not a problem from your nas. If the download is forbiden above 2Gb it's because your DD on the computer where you want to download is in FAT32 and not NTFS ou EXT3 or EXT4... FAT32=2Gb max! the problem don't come from a protocol. it works on FTP, CIFS or Webdav.
The streaming is available by UpNP (ex DLNA). Your nas can do it but this protocol don't work on the net. but you can download your movie by FTP for exemple on you phone, for exemple, while driving and watch it later.