Routers for garage area

You can only have 1 router connected to a modem because modems have only 1 WAN port.  You can, however, as Mouth suggested, connect another router to the 1 router already connected to the modem.  Make sense?  Modem > Router-1 (Gateway) > Router-2 (AP).  Connecting the 2 routers with a cable would be best and while in your garage, devices would wirelessly connect to Router-2.

You could always Install a mesh system such as ORBI, EERO, and the like.