A70 screen mirroring to Amazon FireTV stick 4K

[gazza35]

I'm trying to screen mirror my A70 to my FireTV stick using SmartView

I've gone to the screen mirroring option on my FireTV stick and then enabled SmartView on my Galaxy A70.  It sees the FireTV OK and has the option to mirror screen.  When I press on this and select 'start now' it appears to connect OK, my A70 shows a connection with the blue SmartView icon on the right of my phone screen but the FireTV screen just goes black and is not receiving any video or audio from the phone.

I've tried changing the aspect ratio just in case it was something to do with that but it doesn't change anything.

Both the phone and the FireTV stick work perfectly fine other than this issue

Any ideas why this is not working properly?