Trying to Control Brightness 2009 imac used as a Display Monitorian & Display Fusion Did NOT WORK

Trying to Control Brightness on a 2009 imac being used as a Display with program app Monitorian & Display Fusion Did NOT WORK

Cost: $ free (in-app Purchases)

Microsoft Store: https://apps.microsoft.com/store/detail/monitorian/9NW33J738BL0?hl=en-us&gl=us

Using an iMac as a monitor / Display
Amazon Basics 4K Mini DisplayPort to DisplayPort Cable 6 Feet Black
(01-25-2022)

Cost: $10.50

Using my 27 inch late 2009 iMac as a Display for my PC.
But also using the iMac as a computer, as well as a display.

Video Card is an RTX 2080 Ti

imac running Sierra, 10.12.6

Monitor displays 2K (2560x1440) @ 60Hz

Amazon: https://www.amazon.com/gp/product/B013PWQPFS/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&th=1