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

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