@Reza You have exactly the same problem as I had when I first found MySensors in march last year.
I got a couple of radio and started to play with them. All worked fine as long as they were not more than 5m away from each other. I dropped the idea at that time.
2 months ago I saw the possibility on the website to build a wired network. I started again. With the order for the RS485 bricks I ordered also some radios but now with antenna and amplifier. The only improvement I got was now up to 7m.
I managed now to connect with my greenhouse that is 12m away but had to place a repeater in between but the connection is no reliable.
The discussion that is now ongoing is not related anymore to a RS485 network. I suggest you start an new topic specific for your radio issue. Or we both did do something wrong or the hardware that we buy does not meet the specification for transmitting distance.
Let me now if you started a new topic.