Getting a second IP address for my Arris SB8200 Modem

I've been struggling with this issue for 2-3 days trying to get a solution. My router can't get a valid IP address from Comcast so no WiFi - which is a huge problem. I just upgraded to a Gigabit internet plan and upgraded my modem to Arris SB8200 - an approved modem. I've been on the phone with Comcast support and talked to 3 different support techs - all were no help at all, just blaming the modem or the router. The router was working fine with my previous slower service, and the modem is connecting to the internet and works when directly connected to my computer via ethernet cable. I just can't get the router to work.

I've figured out the problem is because I'm trying to use both ethernet output ports on the modem - one to my computer for maximum speed, the other to my router. Apparently (which no one at Comcast seems to know or understand) this will only work if the ISP can provide 2 IP addresses.

My other option is to connect the modem to the router, then connect an ethernet output on the router to my computer. But that cuts my wired download speed from 800+mbs to about 350mbs. Yikes! That's a BIG hit.

I give up with Comcast phone support on this issue - it's just a waste of time getting nowhere. I've seen that several other people have been able to get help on this same issue through the forum so I'm trying that out of desperation! Thanks for anything you can do.