MS60 - How to add an extra sattelite

I have a mesh system already set up with the MK62 and 2 x MS60's. All working well. Was having Wifi issues out in the yard in a studio and decided to buy another set of devices to use the MS60 to add to my existing network to further extend my coverage. 

When I power up the MS60 and go to the Nighthawk app, i select 'Add sattelite'. It goes through the process, requests that a press the sync button, all works ok, the sattelite settles the LED on a solid blue line but the app reports that the satellite cannot be found. 

Any suggestions? Am I meant to do something else first? 

Thanks

Jo