30% is really low humidity for this type of plant! That's probably what your problem is. There are a lot of ways to try to increase humidity in the area. If you can cluster other wet plants around them, like banana, or grow them around a small pond, that might be helpful. I find that using coconut husks /coir as mulch really helps keep the ground moist for longer, and increase humidity around the plant, if you have access to coconuts. Misting might also be good, but I would say you're going to be fighting an uphill battle with this per your location. I think protecting them from wind and sun will help, again with other plants is probably the best way. I mean, they're not dead, just a little cranky and wilty, so some adjustment might help just fine.
If your home is close enough to the sea you can also devise methods of desalination irrigation, old-school using the sun, a container, and some hoses, to help increase water amount and lessen the cost of using tap water. I don't know but I'm guessing you get really good drainage, like I do, so probably rot from overwatering isn't a concern. I know for me that's impossible to ever do unless we have actual standing water from a flood, because the ground here is mostly coral rock, and I think you said the desert there is also mostly rock...
But just eyeballing the photos, it doesn't look like the planting is dense enough to protect the plants from sun and wind so their leaves can hold moisture in. If you also have excellent drainage, that means that the plants' humidity level is probably just very low for such a tropical plant. Surrounding it with some other plants that like moisture as a wind buffer and for additional shade, and using good mulching like coconut to hold in deep waterings for several days at a time, might help them a ton.