Diagnosis of a FOB transmitter requires the use of a Radio Frequency Detector. Other than battery replacement, the FOB cannot be adjusted or repaired. If damaged or ineffective, the FOB must be replaced with a new unit.
I had to replace the battery in mine a couple of weeks ago. NOT DETECTED kept popping up and I had to start the car with the FOB pressed on the start button. After I changed the battery I still had the same problem so I disconnected the battery for 15 minutes. I have had no issues since. For me, disconnecting the battery worked. Maybe it reset something.
I'll throw in my odd side note as there's always one in the bunch. My fobs will not work with any battery other than the brand (Panasonic) they came with from the factory. I tried 4 or 5 different brands, verifying their voltage with a meter, even the dealer was stumped. Makes absolutely no sense at all.