I have the same keyboard and optical mouse (both are rebranded A4Tech products); it's part of a tieup with Lidl.
The first point is to ask, when you set it up, did you read the instructions?
If not then you will be unaware that you have to press the button under the mouse and then click the button on the receiver. You then press the button under the keyboard. Must be done within about 25 seconds which is more than enough time.
If this doesn't work then install the Silvercrest drivers and repeat the routine.
Apart from the Silvercrest configuration icon in the Taskbar, there are some Advanced mouse settings from Device Manager>Mouse>Properties>Advanced tab.