Comcast wireless & hardwire

We had Comcast install a router/modem to use wireless in the garage/office. Signal has been sporadic at best, even though only about 30-40 feet. Now I want to move the location of the house computer to a farther room.

Questions;

1) Can I just buy a modem & plug the house computer into the cable?
2) Can I do the same to the garage computer?

Both the house & garage are cable ready & have TV. Any specifics I need to know?

Thanks much,
JQ