If the flight controller/drone lose connection to your radio transmitter easily, you could troubleshoot the issue according to the following.
Set the mode to DSM2
This is only for the F3 FC board with native DSMX receiver and OSD, or the BNF drones with this boards. With this FC board, most radio transmitter will get a great range in both DSMX or DSM2 mode. If you occur short range issue with this FC board, switching the mode on your radio transmitter to DSM2 will resolve this issue. Thanks for MMW's video.
Keep the FC Board 0.5m Away
This is only for the F3 EVO FC board with native long range Frsky rx. Because integrated of PA, it brings the famous problem called "swamping". The radio transmitter (Frsky X9D Plus) will warn "telemetry lost" constantly if it is too close to the FC boards. It does not affect control. So keep your radio at least 0.5 meter away from the quad as usual.
Run the Antenna Upwards
Some pilots would complaint that it is definitely short range FC. It looses signal with my transmitter far too often and for what appears to be no reason. This issue is often caused by the location of the antenna. You should keep the antenna away from the FC board to get a stronger signal. For example, run the singal upwards. As show in the image below.