ISP DHCP ERROR

Hi, i've seen this question a few times but in order to private message someone you have to post a question. This is for @ComcastChe I saw you have resolved this issue with a few people now.  I have 600mbps but I can not break the 250mbps and when I reboot my modem, per tech support, I get the ISP DCHP not functioning properly. Any help will be greatly appreciated.