Hello
This is because of post plaster cast stiffness and some residual pain because of ligament injury which take good time around 6-8 weeks or in some cases upto 10-12 weeks to recover .
Next Steps
Go for Physiotherapy do ankle Range of motion exercises , stretching of calf muscles under the supervision of physiotherapist.
Take Ultrasound therapy and IFT for pain Management.
As pain start settling start weight bearing strengthening exercises , balance and proprioception exercises
Kinesiology taping can also be done it helps giving stability to joint and managing the pain as well.
Health Tips
Moist Hot pack pre mobilisation for stiffness.
Cold pack post exercise regimen to prevent any inflammation .