RTAC1200g+ Download master

[Craig A]

I am going dilly trying to add the download master function
user guides all suggest its supported.
https://static.raru.co.za/pdf/521620...ser-manual.pdf
a gazillion websites show that asuswrt is cool and make it so easy to enable,
https://www.asus.com/us/support/FAQ/1016385/

but on mine, the option just doesn't exist
https://ibb.co/gDK085

speaking to asus support, the guy thinks I am mad. I am trying to explain to him that I am missing the button totally. buthe doesnt seem to undetstand me

note: I have a 32gb usb formatted as ext4 plugged in and recognised.
I see the odd thread online about this issue. but no formal answer. I have seen some crazy complex ones about telnetting in and installing packages but I think those are for merlin which isnt supported by my device.
Any advice please?

Thanks