I've stated this dozens of time, Gina, but I'll say it again. In hot conditions, with potting soil that is well-draining, and of course, having the necessary drainage holes in the pot, overwatering is virtually an impossibility. We who grow plumeria in the Deep South, and even though we have unrelenting high humidity here (the relative humidity is a whopping 95% at this very moment), still depend on adequate rain/watering to keep our plumeria healthy.
@LeenaY, my diagnosis is dehydration and heat stress.........which go hand-in-hand.
@tapla, even with our high humidity, and though our high temperatures here are (only) in the low 90's, I have to water all my potted plumeria almost daily. Growing plumeria in SE Florida and growing them in a temperate climate such as you have, is night and day different. I know this. I grew plumeria in northern Mississippi for 25 years and now grow them in southern Florida (5 years).
@LeenaY, I agree with Gina. Move your plumeria so that it/they are getting shade from about 11:00 AM until about 4:00 PM. Do your best to keep the potting soil slightly moist, but never soggy. The proper media will assure you of not having soggy soil. Plumeria are tropicals, not succulents or cacti, and though I've got lots of customers that grow them in desert conditions (southwest USA), they devise ways to increase the humidity around their tropicals. A mister placed strategically, can do this for you. There are other ways to increase humidity as well. Recovery will take weeks, so you'll have to be patient.
