HARDWIRED ALL THE WAY ! ! !
Hard connections are MUCH more reliable that wi-fi systems, what i would do is take off the phone outlet plate and see what they used, if they used a cat5 for phone wire then replace the phone end with an internet punchdown and do the same on the other end, that would create a hardwired connection straight from your modem to your PC. The best possible solution.
If there is not a cat 5, i would recommed picking up an access point that has mesh capabilty and installing them in your house to have a decent internet connection up there.
My Internet has been pretty reliable. However, my PC is in a different room (upstairs) from my router and modem (main floor), and I'd like to keep the PC in the room. The router and modem work fine, but when I move the router and modem to the PC room, the Internet doesn't seem to work. There's an unused phone outlet in the PC room. Would using a phone to Ethernet adapter allow me to connect the PC to Ethernet? If not, what would the best way of connecting to Ethernet be.
If there are coax outlets in both rooms, MoCA adapters may be a solution for you. You can Google "MoCA adapters" for more info.