And Cat 5 ain't an option, sadly.
Well, then your only other option might be to get your hands on some business-grade WiFi kit and play around with that ? It might have a few additional config options. But even that's not guaranteed. There's very little you can do about thick walls, electrical interference and all the other stuff that causes hassle for WiFI. Have you attempted at least a basic WiFI survey of your property ?
Rather than just one additional hotspot, have you thought about an additional intermediary one ? Located somewhere equidistant between the two.
Proper structured cabling still has a role in this day and age, and will continue to do so for the foreseeable future. All the the best multi-hotspot networks all run off a foundation of structured cabling.