[werk_ag]

By the way, try with 16cm and not 18cm since it's near from 1/4 wave length for 433Mhz (if I was not mistaken in calculation, the best measure should be 16,5 cm). Being that "far" from the right value, will affect specially RX like you're experiencing.

Theoretically those modules are 433.92MHz which leads to a 1/4 wave antenna with 17.3cm, but using SDRSHARP I saw that the frequency is normally below that value, around 433.7Mhz.
I don't know about SDRSHARP precision, so this value may not be exact.