Your PBX IP is 137.135.128.253 and the subnet is 255.255.254.0
therefore, you can use any IP address from 137.135.128.1 thru 137.135.128.254 providing its not being used for something else.
Generally .252 is the secondary_enet on the PBX, so don't use that address.
Run the confiugration wizard in CP and make sure the PBX ELAN IP and Subnet Mask is correct in the CP and also assign a valid IP to CP ELAN interface. reboot the CP after you finish the config wizard and you should be in business