After I open it I found that it contains BH1415F chip
This chip has integrated filter that cut everything after 15kHz, and I decided to turn off this filter by removing two capacitors on modulator board. You can find this chip under metal shielding plate.
Here is the picture after modification done.
After that modification it started to pass through full MPX signal and RDS subcarriers.
Note: Don't forget to turn off stereo on your transmitter!
Any questions? Feel free to ask here.