The solution is another radio. The 500mW Sik radios are cheap noise ridden craps. Get a 100mw one without the power amp, those supposed to work more than 500 meters.
Other things to try : Disconnect CTS/RTS wires, no need for them. power radios from separated power source, not from the FC. Check your antennas, get somebody with an antenna meter, usually the crappy ones that come with those radio are NOT tuned for 433Mhz. Also check output power with a power meter.
You can try lower power setting, perhaps full output power overdrive the power amp stage.
But frankly, the best solution is to get rid of them, get a 100mW one or if you want long range get an RFD900 or RFD868 modem pair.