Minimum input voltage

we are planning on using the RAK811 to extend an existing device with LoRa. So far we have primary cell (3.7V-2.8V) and a DCDC to 1.8V. RAK811 asks for 3.15V - 3.45V.
How come this narrow input voltage range? Is there a similar module that can accept 1.8V? Can we simply bypass our DCDC and connect directly to the primary cell (ER14505)
Best regards