What are folks using for Dynamic DNS (no-ip) client?

I haven't used it for a while, but DDClient worked for me.  If your router supports it, though, that's typically a better way to go.  See the configuration that worked for me in this thread:

 DDClient-by-WhoCares-I-hope-this-one-will-work.

That was when I was still running RAIDar 4.2.x on a Pro6, but I think it should still work.  Lots of guidance you can Google, too.

I use no-ip with my ReadyNAS NV+ and it works fine.  My ISP (BT in the UK) does not offer Static IP addresses unless you pay them a fortune!

My router has a dynamic IP WAN process, which checks the WAN ( outside IP) addess and advises no-ip.com when it changes. You configure it for Dynamic IP and choose your supplier, enter username and password. I think this is a standard feature of Routers.

My ReadNAS NV+ has a local fixed IP address 192.168.0.30 and I have set up a port forwarding to the ReadyNAS and it works fine. 

No special software needed for the ReadyNAS.

Try DNSOMatic.  It supports more than one "client" and registrar.  You can have them all sync together or not...  I use it in my Asus Router DDNS update.  You shouldn't have to run it on your NAS but you can as a script if you prefer.  Router config should be easier as I think it's a standard feature these days in modern routers.  Then as the post above indicates, use port forwarding to your NAS.  There's some DNSOMatic plugins too I believe... see: https://community.netgear.com/t5/Community-Add-ons/DnsOmatic/td-p/675811