When exactly do you need this finished by?

If you really wanted to, it’s possible to turn a Pi and rtlsdr into a receiver, but all of the additional RF components won’t be cheap. You’ll need a bias tee to drive the LNB. The bias tee from the V3 dongle only goes to 4.67V. There is 2V of drop out on the LNB and the converter in the LNB doesn’t really like to go below 4.5V (though it will work below that figure).

You’ll also need an additional down converter, since the IF from the LNB will be too high for an rtlsdr. And then you’ll need to demodulate LoRa packets in software. It’s actually all doable, but a ton of work. Probably the easiest thing to do is borrow a Dreamcatcher.