I've just purchased a Nighthawk M6 Pro and wanted to add a AC1200 or similar wifi range extender t

[Boofa]

I have just purchased a Netgear Nighthawk M6 Pro and wanted to add a AC1200 or similar Wifi range extender to it
I have been told they are not compatible so are there other solutions to extend the wifi range?

[plemans]

I wouldn't add an extender. Reason why is hotspots have a limited wifi output power because they're designed to travel. and they don't want them causing interfering with other travelers/signals. so they reduce the wifi output. 

You'd be better to get a router and wire it in. Then either put the M6 in passthrough or the router in access point mode. 

That'll give you further broadcast power and higher speeds than adding an extender. 

[JT240Z]

Agreed.  Extenders cut wifi bandwidth by half.  I would first try using the hotspot in In-Home Performance Mode with the battery removed and the unit plugged in with the supplied charger.  If that doesn't work, then I would consider adding an access point and plug it into the ethernet port of the hotspot.