You don’t need a gym to workout, even with resistance training. To work your shoulders, Dr. Brett McIff with the Utah Department of Health says you can use something as simple as a couple of books to build strength and stability in your shoulders.
Sit in a chair while holding on to a book in each hand. Make it something with some weight, no novellas or short stories will be enough to challenge your muscles. Keeping your abs engaged and your posture tall, raise the books with the palms facing each other from hip height to shoulder height. There isn’t really much benefit for raising them any higher for this movement. Slowly lower the books back to the start position. If you find this too challenging, you can put your back against the chair back for stability. Repeat 8-12 repetitions, 1-3 sets.