Being denied extension of return to work program

Hello.

I work for Canada Revenue Agency and was diagnosed with breast cancer in 2023. I finished treatment and have been in the process of returning back to work. My claim is over the 24 month mark with SunLife and just recently my family doctor wrote that I am not to work for more than 5 hours a day due to fatigue and that working more than 5 hours will cause further sickness. I have requested a extension of my return to work program and have been denied. What should I be doing?