"Your ISP's DHCP does not function properly"

I'm here as a last resort. I've set up three appointments for a tech to call. I've waited 48 hours now to get a call only to be told twice that there's nothing scheduled and I should just reschedule. The online assistants cant help. I've called the Xfinity number 5 times now. Currently I've been on hold for almost an hour and a half.

I just bought a new router ASUS RT-AC88U and modem Arris Surfboard SB8200 to handle Gigabit speed. The modem works fine, but when I try to connect the router I have the error "Your ISP's DHCP does not function properly".  Does anyone have any ideas of ways I can solve this on my end so I can stop waiting for a phone call that apparently is never coming?

Much appreciated,

Colin