@sky75 they have provided to me some beta build which fixed the issue. :)

  @sky75  I still have the issue with the MR600, but I'm not prepared to use a beta firmware build as @iSnoopy suggested.  I need to ensure the router is stable and fully supported, as it is in a commercial environment, so I will only use the official, stable releases. 

It is not really a problem for me any more though. Since installing, I have noticed a pattern:

- When there is nobody in the building, and therefore no WiFi devices connected to the MR600, the router usually appears offline in the Tether app.  Even though I know it is really online because I also have an RE330 extender there, using OneMesh, and the RE330 is online in Tether.   

- When there are people connected to the WiFi and actively using it, the router always seems online in the Tether (remotely).  Or looking at it the other way around, whenever it does show "Online", it's because someone is definitely connected. 

- If I am there and connected to the main WiFi network (not guest) then everything works as expected with Tether showing the MR600 as a local device. 

This is fine - when nobody is there then I wouldn't need to do anything with Tether anyway.

I was just concerned initially, before anyone was using it, and it would always appear offline.   You might find that it will work just fine once it is being actively used. 

I am guessing maybe the router goes into a slightly dormant state when there are no devices connected, and no packets being routed, perhaps for power saving. Or maybe the functionality to "dial home" to TP-Link is somehow dependent on devices being connected, or traffic generally flowing.  I don't know, but my theory is that if I had some other device in the building that was always using the Internet, or maintaining TCP keepalive, then the router might remain "online" in Tether.    I'll update this thread (if I remember!) when I am in a position test that theory, maybe later this year.

Hope this helps, or at least stops you worrying about it!