trigger a function of a Arduino Uno from signal received by LoRa module
I'm assuming you have read the datasheet for the SX1278.  If not, please refer to it at:
SX1278 Datasheet
On page 69 you will see a couple of tables regarding the functions possible on the digital I/O (i.e. DIO) pins:

I'm not clear what exactly you are looking for or which mode you are operating in (you have provided almost no information about your project) but all the information you might need is available on these DIO pins.
You can connect the appropriate pin or pins to some of the edge sensitive lines on your Arduino and define and enable an INTERRUPT in your code.  So when the event you are waiting for happens, an interrupt will occur, you can go and read the data and do whatever with it.