Certified GKH Gardening Expert
It would seem that you are on the very edge of where they can survive outdoors. You will come to expect quite a bit of stress to the plant, since it will deal with climate during some portion of the year that it doesn't do well with.
With that being said, there are several things that will cause what you are seeing. Usually these symptoms accompany overwatering, or too much humidity. If you are noticing brown or black spots that spread rapidly, then it will be in need of treatment. If it is just curling, then you can chalk it up to the shrub defending itself against the elements.
This article will help you to care for these: https://www.gardeningknowhow.com/houseplants/rubber-tree/rubber-plant-outdoors.htm