After login to NoMachine Network or adding a new NoMachine Network connection on an Android device, the status of a remote machine present on NoMachine Network is shown always as offline. 

As a consequence, there is no possibility to connect to that machine from an Android tablet, however it is still possible to connect from Android phones even if the machine status is wrong.

As a workaround, restart the remote machine or remove and re-add it to NoMachine Network: its status will be then displayed correctly on Android devices.