Hello,
This is usually a hardware issue with the keyboard. Underneath the keyboard, there is a circuit board with numerous signal tracks. Cracks in the tracks, dust or debris buildup, or corrosion can disrupt the signal flow.
If the USB keyboard works, that only proves further that it is hardware issue. Unfortunately that's not a Microsoft issue and it is beyond what this forum can help. You need to contact Dell support, or take it to local repair place.