Everything I have been able to gather state that the max distance is 1200 meters for 3997 feet. In that respect you are good. What the issue I believe your having is the bawd rate is not correct for a particular distance. The shorter the distance the higher the bawd rate should be. I know that looks off but if you dive into the manual for one of our 23X analog cams you will find it in the appendix.
APPENDIX II ABOUT RS485 BUS
RS485 Bus Main Feature
RS485 is semi duplex communication cable of impedance 120Ω. Its max load amount is 32
effective loads (including main control device and devices to be charged).
RS485 Bus Transmission Distance
When we take 0.56mm (24AWG) twisted-pairas communication cable,the max transmissiondistance (theoretically) are listed below(according to different baud rates).In the following situations, the max transmission distance shall become shorter accordingly: The communication cable is a little bit thin; The surrounding environment has strong electromagnetic interference;