Well… first off you can’t do either so stop right here.
To have your modem put in bridge mode you will have to call Comcast customer support. They will do it remotely for ya no questions asked. Once it is provisioned then you can logon to the modem and enable bridge mode.
Your modem can also assign an IP to your router in router mode. To put modem in semi-bridge mode go to Gateway, Firewall and select Custom Seruity. Click on the link and select “Disable entire firewall”. Now, go to Gateway and click on VIEW CONNECTED DEVICES. Note the IP address of your router and then go to Advanced, DMZ, enable DMZ and put your router’s IP there.