ASUS RT-AC1750 router won't connect after replacing my modem "ISP's DHCP does not function properly"

I have seen previous threads that have the exact same problem that I am having. Comcast recently did service work in my area and after that my internet speed reduced drastically and my modem kept disconnecting. I obtained a new personal-owned modem yesterday and it works great, but now my router won't connect to the internet to give my house WiFi access.

I saw in previous threads that they were able to resolve the problem by having comcast "send a provisioning signal to the modem." I had a service tech at my house today but he didn't seem to understand the problem.

I have tried multiple cords, multiple power cycles of the router and modem, factory restarts, etc. and nothing is working.

I have a ARRIS Surfboard SB8200 modem and a ASUS RT-AC1750 router.