Plumerias do best in full sun, good draining soil, and moderate water during the growing season, with drier conditions during winter. Plants in general are not able to fend off diseases and pests when they are not provided their optimum environment. The spots suggest a possible fungal disease which would result from the plant not being able to have the optimum environment (full sun). The mottling of the leaves sounds like a fertilization issue. You will want to feed with a slow-release or organic fertilizer formulated for blooming plants during the growing season. As it get cooler, your plant is due to lose its leaves. In order to prevent the spread of any diseases or pests that might be present, you will want to collect and dispose of the fallen leaves. We have had some cool temps lately and Plumeria do not tolerate frost . You will want to provide protection as we go into the winter months.