Archie already suggested that you can use the form's Load event to display the error on a standard Label:
Then do it in your form load event:
Private Sub MainForm_Load(sender As Object, e As EventArgs) Handles MyBase.Load
If ModbusTCPCom1.DisableSubscriptions Then
Label1.Text = "Subscriptions Disabled"
End If
End Sub
Instead of putting "Subscriptions Disabled" you can just put your text (and also Set SupressErrorDisplay to True on all AHMI controls).
If you add a timer to the form then you can continuously check the DisableSubscriptions property to see when it changes to False so you can remove the message and if necessary disable this timer (you could also use this timer to initially set the message instead of using the form's Load event).