s33 comcast not interbet

Hello all. 
I recently purchased an s33 to replace comcast modem.

using the surfboard app and Xfinity app, I was able to get the s33 modem connected, blue light and the Xfinity app showed it. 
 

I didn’t have internet though.

so it got me thinking, I need a router. 
I pulled out an old Cisco router I have, connected port FE0/1 and set it for “ip addres dhcp”, it won’t obtain an ip.

after hours of frustration, I connected the Comcast modem back up, Cisco router obtained an ip from it.

what am I doing wrong? Missing?

thanks.