The gateway address is wrong, or at least, wrong for this network. The subnet mask defines the network portion to be the first 8+8+4+0 = 20 bits from the left. The network part of the PLC address is hence 172.16.0, but the network part of the gateway is 172.16.240, that is, different networks. If you don't need the gateway, leave it out. If you do, the PLC has to be in the same network as it. I'm not confirming this is the cause of your problems, however you have seemed to indicate the gateway address is invalid for this PLC addresses, and you are correct.