Good question, I found my self asking the same thing, hence why I put the 2.8v into the HV pin, thinking, wrongly, it would set the voltage.
Had a look over the PDF, looking at the digram I think I need to have 5v going into the HV and 2.8v from the buck converter going into the LV pin (this lets the shifter know what voltage we are shifting too?), then at the high end have the RX and TX from the arduino and at the low end the RX and TX going into the SIM800L.
Have I understood that right?