Starting from GNOME 3.26, the drawer showing 3rd party application icons was removed. As a consequence, it's no longer possible to display the !M icon out-of-the-box. This is not a NoMachine issue, but a behaviour introduced by GNOME 3, please see also:
https://blogs.gnome.org/aday/2017/08/31/status-icons-and-gnome/

In order to workaround the problem, it's possible to install the following GNOME extension:
https://extensions.gnome.org/extension/1031/topicons/

We suggest to consult the official documentation of your Linux for how to install it.

As an alternative, you could choose to adopt a lightweight desktop which doesn't exhibits this behaviour, for example MATE or Xfce.