It's all about impedance RFM has on its output. Needs to be 50 ohm, so that antenna can radiate max power (max power also delivered from RFM at that point), everything else means your antenna is mistuned and RFM won't be able to transfer the power onto antenna.
For your antenna, do you have bare wire or insulated?
Your 78mm is close, but probably a bit long. Not that if you have insulated wire, you will need to have it a bit shorter than bare wire.
https://lowpowerlab.com/guide/rf-best-practices/velocity-factor/try 74mm, or even better, set your module to lower freq., all the way to 890MHz. (I assume your antenna wire in insulated, and you have 78mm, which would be right for bare wire @ 915M, but also right for insulated wire @ 868M)