Repeating 5Ghz is difficult because it really only does work at a shorter range - 2.4Ghz is what will actually cover a good distance. I'd suggest trying to move your router to a more central location that can cover the house better for everyone, or trying to get an ethernet connection up there - it can be a lot easier than you'd expect. If you happen to have coax connections up there or close by, you can use MOCA adapters as well - these convert coax to ethernet and let you reach other places in the house without having to run more cabling.
P
pillar
this could be a good option for you too, to get a second access point up.