WIFI antennas are designed to spread horizontally not vertically. If your router has external antennas, you could try angling one at a 45 degree angle to where you need more coverage. If that doesn't change anything, then your only option is to add another WIFI source via an access point. An access point is an independent WIFI source that is connected back to your primary router via some wired network. If you can't use ethernet cable, then powerline network adapters are usually then best choice.