Have to change my ip adress for security reasons but trying the "ipconfig/renew" command prompt method doesn't work.
Which IP address are you referring to? The public address the Comcast network assigns to your modem/gateway? The 'ipconfig/renew' option only 'renews' the existing address for another 24 hour period. You might try the 'ipconfig/release' command to get a new address, however for a PC/laptop/device, that only changes the address on your local network, usually a private address.
That thread suggests powering down the modem and router for 1 minute, and powering up the modem first, fully synced, then the router.
Might not be your exact scenario though.
Powercycling the modem won't result in a change of the public IP address. Neither will performing an IP release / renew if one is using a router. That would only release and renew the LAN / private address...
OP. If you are using a router that has a MAC address cloning feature, try cloning a different MAC address in to it. This will force their DHCP server to issue a different public IP address to your connection.
The Comcast supplied gateway devices do not offer a MAC address cloning feature.