No. If you are using the target host's IP address to connect to a NoMachine server, a Network Subscription is not necessary. You need a Network Subscription if you want to connect to the target host using its Machine Id i.e you are connecting through the NoMachine Network service. NoMachine Network allows you to connect to any Internet host without knowing its IP address, behind any firewall or NAT. 

A Network Subscription must be activated for your NoMachine account to be able to reach a NoMachine host published on NoMachine Network. The target machine can be running any NoMachine server software (free edition or one of the products from the NoMachine Enterprise suite).

Also see:

Can I continue to use the NoMachine free software after trialing NoMachine Network?
https://kb.nomachine.com/AR01X01342

How does the NoMachine Network Subscription model work?
https://kb.nomachine.com/AR09V01249