yellowing leaves (sometimes yellowing starting at the leaf tips) is completely normal for adenium, as the uptake in water has or is about cease....adeniums in cooler temperatures will go dormant & leaves falling off is the start of dormancy ...
some plants may loose all its leaves, some will drop about half, while other plants may retain all of its leaves
keep a close eye on the stem tips ..... during dormancy, stem tips will appear "bald" with no new growth or tiny leaves emerging from the stem tips ....when you see bald stem tips, stop watering and stop adding fertilizer
adeniums can handle the heat - but they can not handle wet cool roots - root rot will result, often killing the plant
plants can go as long as 180 days without any additional water or nutrients - plants will get what it needs from the caudex
have a look at these 2 guides to get you through the winter months
https://adenium.tucsoncactus.o...
https://www.desertmuseum.org/v...
