That is not the case with this keyboard, on my Asus laptop the function keys behave as expected, e.g. F1 is Help, F10 pressed with Shift is the application key menu, Jaws Key + F7 brings up the list of links etc. and there is no need to press the FN key.
When I received my K780 the function keys were all doing their alternate media control functions, e.g. F10 is Mute, F11 volume down, F12 volume up etc.
Tom, after installing Logitech Options you have to run the software, if I do this I start out on the Keyboard tab, the other tab in this app is the Easy Switch Tab where you can see which device you have on F1, which is on F2 and which is on F3 (provided you have 2 or 3 devices paired).
Here is Jaws speech history when I start tabbing:
Keyboard Tab selected
I think I had to tab 10 or 11 times to get to the “Use F1 - F12 as standard function keys” checkbox, in your case this is unchecked, press the spacebar to check it and then you are good to go. You may have to tab a bunch more times to see if there is an OK button, can’t remember if there is or if you can just close the application.