The device in the image is a zener diode. Please see my recent posts for an explanation about its role.
I don’t recommend the Waveshare D. The Vmp is set too low (4.6V). This is important, since the IV curve for silicon solar cells is highly asymmetric, and the low side is very steep. You could do surgery on the PCB to raise the Vmp, but I opted to just go with the DFR0559 instead.
I recommend that you not try to charge the battery using USB on the RAK19007, since the power-dissipation specification for its charge-controller chip is too weak.