Given that is your use case, wouldn't a simple Switch showing you that the alarm is up and running more appropriate? You can use a rule to integrate the Items that tell you when the netamo is up. I do something similar with this.
My UI philosophy is that it should be explicit and simple. To me, having to look at a presence field and interpret that to tell me whether the netamo is up is more complex and less explicit than a Switch telling me it is up or not. Let a Rule do the interpretation for me.
So, for me, I have a global online group that I put on my sitemap as Text. When all my devices are online it shows ON and all is good. If any device goes offline or hasn't been heard from in too long of a time, the Group goes OFFand I then can bring up the admin sitemap to see what device it is and debug from there.