Hi,
Just made a drone with Racerstar StarF4S board. Everything work fine exept OSD
I connected the RX TX cables to RXD and TXD pads of the GPS part.

Tried changing order, enabling telemetry.

Maybe I connected it to the wrong pads, but I don't see any other RX/TX pads.

Did you make sure to enable Configuration/MSP for the UART? Also, I had this very same issue and have spent the past few weeks with DJI support trying to fix it. Turns out I may have had a bad Air Unit. You may want to start the conversation now with DJI support because their warranty is only 15 days from you receiving the part to get a replacement for issues like this.