Maximum PC

The background just in case you see something else that can be fixed: I've moved into a rental home and is my 1st two story home and is a really crowded (and crappy HOA community). What I'm facing is the herd of garage PC's having really iffy connections and constant drops. I use to get by at the last place with a DIR-655 and could fire right through the garage wall but there were only perhaps 4 other wireless networks in the area where I now have no less than 15. Thinking I have "bad" neighbors I finally got a dual-band router and went ahead with the new AC spec with an Asus RT-AC66U. Thing is that I can grab either of these routers' signals blocks away but I just have a black zone in the garage that they just can't reach with any sort of reliability.

So...I'm wanting to just run a pair of CAT6e out here, run the garage computer off it and then use the other line for the DIR-655 for the rest of the little buggers and get folding again this Fall/Winter. Problem is, I've never wired a two story home and know jack about how to get a cable ran in the 1st floor ceiling inside the house. Seems like that if you don't do wiring of any sort before it's drywalled you're hosed trying to do old-work add-ons like this or even basic Romex runs. Are there open channels in ceilings/floors like in interior walls or are there kickerboards like exterior walls that I'll run into? I'd pay for someone to do this, but this ain't my house, I'm looking to get us out of here in a year, and don't want to roll a bunch of money into something simple like this for someone else.

I see the exposed bundle of Romex A/C cable coming in to the garage from the house, but that's my only clue. My brother-in-law is an electrician, took a quick look, and he just told me it's better to try wireless than fuck with a 2-story home. Welp, I've tried that and this small area of hell is Radio-free Europe. So...any of you know a good fix to this situation/know how cheaply built two story homes are built so that I can get a good fish-tape run and a good pull to a Cali coded garage? And it IS cheaply built...the gable vents look like they didn't have a saw and just hammered the particle board out and slapped the thing on the outside and called it beer-thirty. Not to mention this entire wall of sockets I'm typing from are on a GFI in the upstairs bathroom. I hate this place so far, so help a guy out if ya can!