Two plates have been installed in both the forearm bone to treat the fracture. When will my forearm regain full mobility, doctor denying physiotherapy?

Need more details. A lot will depend what else was injured besides the bone i.E muscles, tendons, ligaments. Also a lot will depend on the exact type of fracture, how old you are, and how motivated you are among other things. Therapy is a key part of many orthopedic recoveries so if you don't feel you are making adequate progress you should speak to your doctor again. Best of luck.
PT. I'm not sure why the physician is denying therapy for your limited mobility unless it is too soon after having your surgery.