Thank you for the additional images.
Your shrub is a hardy hibiscus which, if planted in the ground would be fine outdoors throughout the winter. Hardy hibiscus is a deciduous plant, thus, the leaf loss is normal at this time of year.
However, because the shrub is growing in a pot, it is at risk of dying during one or more of the frosts that are yet to come. Frankly, that might be a good thing because then, you could start over with a healthy, fresh, plant next spring.
Do you see all those small bumps, both white and dark, on the backs of the leaves, also on all the branches? They are scales, small sucking insect pests which will gradually weaken the shrub and may kill it.
Then, too, the sticky stuff on the leaves and branches is honeydew, the excrement shed by scales as they feed on fluids they suck out of the plant.
Scale is very difficult to eradicate, especially on an indoor plant. You could start by hosing off the shrub to get rid of the sticky stuff. Then, continue by physically removing the scales from the leaves and stems. Use gloves and/or a paper towel or cloth to rub them off.
The bottom line is that it's extremely difficult to get rid of all the scale because they hide in cracks and crevices you can't get to. Applying a superior horticultural oil will help to eradicate the scale but will need to be used several times, according to label directions.
Frankly, I encourage you to chalk this up to a learning experience, then save yourself a lot of grief by getting rid of the plant now. Then, if you've enjoyed having this plant, you could start over with a healthy, fresh, plant next spring and plant it in a sunny spot in the garden where you can enjoy it.