sorry to hear about this - in the end @latigid on will be able to help best, as he designs all the hardware and knows everything best - but here are some ideas upfront:
a) Could you try another USB cable? You would not believe how many USB connection problems we experienced because of either broken USB cables or ones that supply power only.
b) Could you check that the Waveshare MCU board orientation is correct and that all headers are inserted firmly?
c) Could you check the two switches on the Waveshare PCB? One sets the power supply mode, the other the boot address, both must be correctly set.
d) Did you install the resistor R101 and the nearby region components? This is required for a proper bootup of the STM32 microcontroller.
e) could you check the soldering around the phyisical USB connector, the TVS diode (is the orientation correct??) and around the bottom of the Waveshare headers? It could be that one USB line is interrupted, e.g. due to an unwanted soldering short or maybe the TVS diode installed the wrong way. Andy could probably give you a pointer which signal goes to which final pin on the STM32 microcontroller - you could trace the USB RX and TX pins all the way from the physical USB connector up to the microcontroller pins to be sure there is connectivity - my guess is that we have an interruption here.
If nothing helps, if you could upload high-resolution pictures of the bottom and top of the core PCB, we could have a closer look.
Best regards and good luck!