you want ipex 1 not ipex 4, most frsky antennas are ipex 4 the tiny ones. the r9mx from frsky uses ipex 1.
https://www.amazon.com/Receiver-Antenna-Futaba-Flysky-Replacement/dp/B0743B56QZ
They are just U.FL 2.4ghz antennas.
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I bought these: https://smile.amazon.com/gp/product/B0743B56QZ and they work.