Humm... Didn't realize that DynDNS went to a paid only model now. I remember them having to do some work arounds to get to the free version six months ago when they only showed the paid only models.
Anyhow, with your QNAP NAS, they currently support the following DDNS providers (Qnap FAQ
For game server hosting, try no-ip.com; a few friends use this for their TF2 and minecraft servers hosted at home. Works pretty good if dyndns isn't an option anymore. And I'm pretty sure you can't use your NAS box to host a game server since it uses their own proprietary OS.