These days the cable modem is the weakest link in most homes. Even if you do enterprise networking the way that Comcast work is pretty mysterious and hard to figure out. Here are some pointers for diagnosing problems.
As an aside they have a push on to get you to rent modems. Makes great sense for them. Take a $60 modem and least for five years at$10 a month and you have some major margin. But the fact is that with a little work to make it happen but here are the steps

1. How to know if your modem does. This is actually a little hard but easiest way is to directly connect your laptop to modem. And then browse to the status page. On Arris SB6141 modems the default is 192.168.100.1
2. Note that if you are using a MacBook Pro with the USB C to Ethernet adapter, on Mac OS X Mojave there seems to be a bug where the connection isn't automatically recognized quite a bit. This requires a reboot of your Mac to make the USB device recognition work.

3. On our breaking Arris 6141, the modem would constantly reboot and this was not good.

4. Make sure you have a backup modem around. These things are only \$60 so cheap insurance.

5. In my case I tried to use an old SB5140 modem and it would activate but would not connect. It kept failing with a bad tftp download. The log page tells you a lot

6. The actual way this works is pretty weird. When you plug a new modem in for a while you get internet access and then it fails. When you connect your computer you are then punched into a special mode where the modem changes and the IP address completely changes. I've never seen this happen with any device but apparently the cable modem has special firmware that allows this.

7. You get thrown into an activation page and then you need to authenticate with your Comcast id so have that handy. This writes your modem and it's special cable Mac id into their database.

8. In my case, they said the modem was old and unsupported and not really allowed on the network (what?). So fortunately I had another spare 6141. Make sure to check on mydevinfo.xfinity.com to see what's allowed. As an aside their authentication is broken so you have to know your zip code and how much internet bandwidth you've brought.

9. Now plugging it back in, this one did seem to work. No tftp errors. But still no access and the authentication failed.

10. Calling Comcast they will do a manual activation. The very important thing is to make sure to say and then repeat your Mac id really carefully. Nothing works if that is incorrectly typed. This happened to me and another hour of life wasted. If the tech doesn't "see" the modem make sure to ask him for the "CMAC" and make sure it is the same as the modem.

Net, net pretty frustrating way to spend three hours! The main thing to do is to make sure you have a backup modem around as life is pretty critical these days to live life without on and they are not expensive.