Any recomendations for low-voltage wires through walls?
The easiest is to get an extension cord for the low voltage cable on the AC adapter. Most likely the adapter comes equipped with a 2.1mm or 2.5mm center pin barrel type power plug. Extension cords for these are readily available. One would look like this:



The extension cable would allow you to power the router from within the same room from an electrical outlet instead of having to go off through the wall into the garage.

My favorite place to get various types of cables, including DC power extension cables, is PCH cables. Very reasonable prices. They have the 2.1mm size in 6 foot and 100 foot lengths. 

You can also find such cables in eBay and at Amazon.

(Note: I am not affiliated with PCH Cables in any way other than being a very happy customer).