i think that u did well. ur gateway (zeroshell) usually will be given the 192.168.0.1 instead of 192.168.0.75 ip on the lan side, that just a common agreement to make things easy.
On the cable modem subject: My cable internet provider ( UPC) requires me to register my MAC address with them, after that they know its me and give me the full speed on the line. Maybe yours does to. I dont know if zeroshel can spoof a MAC address, if not just call ur ISP and ask them if they require it and if so give them the new MAC address. If its the case, there is usually in your manual of the cable modem a line that says go to this or that website and enter your details. DHCP is how my gateway gets an address from my cable modem.