Sim800c Bluetooth very low anntena range problem on different modules!

I have designed a board with a SIM800C and I have followed all details with its given datasheet. Everything is working well except the Bluetooth that gives very low antenna strength (less than 10cm.)

Have you had this problem? What can be the reason?

Things that I have tried:

1) Three different modules.
2) One SIM800C wired on the air to see if some signal collision is happening on my board
3) Connecting and trying 3 different WiFi 2.4Ghz, 50ohm antennas directly to the BT pin of the SIM800C.
4) Adding almost random capacitors and a few random resistors between/on antenna and BT pin.
5) Have tried other BT module's antenna by directly wiring it to the SIM800C BT pin and disconnecting its path from the original module.

None of these even helped to increase to 50cm of BT antenna strength. I do not understand it at all. What I am missing? Do I need to add a specific resistor or capacitor to make it work? In the datasheet, it says it is usually zero without any capacitors.

Also, note that the firmware has been updated from the given files from the original website,
I am not an expert with antenna stuff and could somebody tell me step by step what do I need to do please! note that GND is connected to GSM by default in the module and works fine. I have 4.1v-3A on my module.

Does no one have any idea?

Hello Saeedvand,
I am also faceing the issue with SIM800C. did you find anything to solve this issue. i am also try to connect 2.4Ghz anteena with 1.5dB Gain.

Dont put any resistor or capacitor, Just have direct connection from antenna pin to antenna. Bluetooth 3.0 on SIM800C has max range of 10 to 15m indoors.