COMMUNICATION BETWEEN S7-200 & S7-300 VIA WIRELESS ETHERNET

Dear,
I have a project as follow:
Two station, distance from this station to another is about 3km (assume station A and station B)
Station A is PLC S7-300 connect to ethernet switch and this switch connect to a wireless device (Access point). Station B is PLC S7-200 connect to switch and this switch connect to a wireless device also (Access point)
I ready configurate for two wireless device and test by two computer by "Ping" statement from thic computer to anothe computer and it is ok
When I connect two PLC to one switch, it is ok
The problem is Two PLC cannot connect together, I use computer with " Ping" statement from computer to PLC and it cannot connect also.
Pls help me sold this problem, Do I have to configurate anything more on two PLC ?

Thanks
Nguyen Phuoc Duy