It can be done through a VPN. The limitation is available upload bandwidth from your server and download bandwidth to your phone, but if you can stream from your NAS with another app then well worth a try.
Heres a guide to setting up your own VPN server.
Edit: One of the suggestions in that guide is to flash new VPN enabled firmware onto a router. I would recommend against that particular method unless you know exactly what you are doing and are willing to risk bricking the router.