Depending on the cultivar of your plumeria, most plumerias are just dormant when it is cold season. As long as the stem is firm, it should be okay. So no watering or fertilizer is given while it is dormant, just let it be.
When your temps are more consistently warm and higher, you can slowly acclimate it outdoors and it will be happier being out in full sun and regular watering and fertilizing can resume.
