First of all, to create two separate networks you should create two different instances (or edit existing one and create only one). Two of them would be LAN interfaces, one's DHCP server giving out IP addresses to wired clients, and another one would be giving out to Wireless clients (you will need to configure wireless settings so it would use your new LAN interface).
Second of all, you cant have subnet 300, as the maximum is 255. IP consists 32 bits and all these 32 bits are divided in 4 equal segments,i.e., 8 bits. In computing world almost every thing is in power of 2. So when we calculate 2^8, it is 256. Since any IP address starts from 0 so it ends at 255.