Hi pne123,
I would strongly recommend discussing your questions with the treating doctor / specialist who put the plaster cast on your ankle in the first place to ensure optimal treatment of your sprained ankle.
Once the treating doctor / specialist has removed the plaster cast and has indicated it is safe to begin mobilisation and weight bearing activity then you should begin mobility exercises and some weight bearing. Usually ankle flexibility exercises and weight bearing can begin early provided there is no increase in symptoms.
I would clarify your situation with your doctor and physiotherapist sooner rather than later to ensure the best outcome.
Hope that helps and good luck!