Find the controller in the Device Manager.
Open device properties and navigate to details.
Select Hardware Ids in the dropdown.
Copy VID (Vendor ID) and PID (Product ID)

Use a Hexadecimal Converter to convert ids from hexadecimal to decimal

Open the following files in a text editor:

<path-to-game>\KellerGame\Config\PC\DefaultJoysticks.ini
<path-to-game>\KellerGame\Config\PC\KellerJoysticks.ini

<path-to-game>\KellerGame\Config\PCKellerJoysicks.ini
<path-to-game>KellerGame\Config\DefaultJoysticks.ini

In each file under [Joystick 0], edit VendorID and ProductID to match the converted decimal values for the used controller
Under the ids, add UseXInput=true

If you have an Xbox Elite Series 2 controller, the text under [Joystick0] in each of the 4 .ini files mentioned above should look like this:

ProductName=XBox Controller HID Minidriver, XID
VendorID=1118
ProductID=2850
Joy1=XboxTypeS_A
Joy2=XboxTypeS_B
Joy3=XboxTypeS_X
Joy4=XboxTypeS_Y
Joy5=XboxTypeS_RightShoulder
Joy6=XboxTypeS_LeftShoulder
Joy7=XboxTypeS_LeftThumbstick
Joy8=XboxTypeS_RightThumbstick
Joy9=XboxTypeS_Start
Joy10=XboxTypeS_Back
Joy11=
Joy12=
Joy13=
Joy14=
Joy15=
Joy16=
Axis_X=XboxTypeS_LeftX
Axis_Y=XboxTypeS_LeftY
Axis_Z=XboxTypeS_RightX
Axis_Rotation_X=
Axis_Rotation_Y=
Axis_Rotation_Z=XboxTypeS_RightY
Slider_0=XboxTypeS_RightTrigger
Slider_1=XboxTypeS_LeftTrigger
DPadUp=XboxTypeS_DPad_Up
DPadDown=XboxTypeS_DPad_Down
DPadLeft=XboxTypeS_DPad_Left
DPadRight=XboxTypeS_DPad_Right
UseXInput=true

After you do this, your controller will work. A few things to keep in mind:

VendorID should always be the same if you're using an Xbox controller.

ProductID can be different depending on what model of Xbox controller you have.

Keep in mind that your PC will read two devices if you connect your wireless controller to your computer via USB. The Device Manager will list two controllers and each will have their own unique ProductID. To ensure that you are using the correct ProductID to use your wireless controller, connect your device via Bluetooth and find the ProductID while it is connected this way.

Cheers!