Run an ethernet cable from your router to the building. It's the most reliable connection and doesn't depend on signal strength. Bury the cable in the same trench as your electrical if you're doing both at the same time.
Use a mesh Wi-Fi system or a dedicated access point. Most modern mesh routers cover a backyard building without issues, especially if you place one node near the back of the house facing the shed.
Use a Wi-Fi range extender. Cheapest option, works fine for basic browsing and video calls, but can be spotty for heavy use. For a full-time office, go ethernet or mesh.
