UOSunshine,
Yes, you can plug the cat5 cable directly into the back of your HR24. No, Comcast does not appear to have issues when you have a wired connection. There are some issues with their wireless service.
You need to do a bit of troubleshooting on your connection. Open a command window on your laptop and enter the command ipconfig. Notice the addresses you have for your IPv4 Address and Default Gateway. If you see numbers starting with 192.168.x.x. you have a class "c" network setup. But, if you have 10 as the first digit of your address you have a Class A private network which Comcast has decided to adopt and your D* receiver may not be able to adjust to getting this address. Before I go further, take a look at your addressing and let us know so that we can get a better resolution.
Bob