Hello Vaxen,
Welcome to Microsoft Community. I appreciate your efforts towards resolving the issue. I will certainly help you with the issue that you are facing.
Any wireless Mouse and Keyboard that is connected to the PC via the USB dongle that comes with them, requires the keyboard and mouse to be in range to the dongle. When you plug in the dongle at the back of your PC, the keyboard and mouse might not get proper signals to the dingle or might not be in proper range.
To diagnose this issue further, I would suggest you to try the following.
If possible, try to turn the tower around in a way that the keyboard and mouse is in the straight range, just as it is when you connect the dongle to the front USB port. If it detects it and works fine, you might have to buy a USB extender cable or device. It will connect to the back port and extend it to be connected in the front.
Do let us know the results. If the issue persists, we can work on the software part and check where the issue is.
Hope this information helps.