Solution #4: Point the antennas in different directions
It may seem a little strange, but WiFi devices work best when the incoming signal is parallel with their internal WiFi receivers. That means that if your router is the two-antenna type (and those work the best, by the way, regardless of cost), you should position the one that points most directly at any stationary devices like desktop PCs and laptops horizontally. Think of it as pointing a magic wand at your computer.
The other antenna should remain vertical. The reason for this is that mobile devices are more likely to pick up the signal when the antenna is pointed in a more universal direction. Give it a try and see if all your devices connect more consistently.