Depends from where you are connecting.
IPv4:
From your own LAN you just use the internal IPv4 of the NAS. Usually neither the external IP nor the no-ip domain work from here.
From the outside you use the current, valid IPv4 of your router (the external one) or the no-ip address resolving to that IPv4 (you know you need to keep updating this IPv4 on their servers by using their client software or router feature, right?). For that to work, you need to configure the NAS FTP server to be accessible externally, for that you also need to forward posts in your router, especially for Passive mode. The NAS manual should cover this, but you can also loosely refer to the Network Configuration guide although that one's tailored to FileZilla Server.
IPv6:
Just use the current IPv6 or an URL resolving to it. You need to use a dynamic service only if your ISP/router randomizes the network prefix.
IPv4:
From your own LAN you just use the internal IPv4 of the NAS. Usually neither the external IP nor the no-ip domain work from here.
From the outside you use the current, valid IPv4 of your router (the external one) or the no-ip address resolving to that IPv4 (you know you need to keep updating this IPv4 on their servers by using their client software or router feature, right?). For that to work, you need to configure the NAS FTP server to be accessible externally, for that you also need to forward posts in your router, especially for Passive mode. The NAS manual should cover this, but you can also loosely refer to the Network Configuration guide although that one's tailored to FileZilla Server.
IPv6:
Just use the current IPv6 or an URL resolving to it. You need to use a dynamic service only if your ISP/router randomizes the network prefix.