'Portrait Orientation Lock button' disappeared from Control Centre on my iPhone

"Screen orientation lock button gone: My Lock Screen Orientation button has disappeared from control center. Does anyone know how to get it back?"

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Adding Controls to Control Center:

You can rearrange, add, and remove controls in Control Center. While in Control Center, tap Add a Control at the bottom of the screen to open the controls gallery. Next, tap a control to add it to Control Center. Then, close Control Center.

Go Here: Use and customize Control Center on iPhone - Apple Support

In Depth:

1. Open: Control Center.
2. Tap:  + at the top left of Control Center to begin editing the controls.
3. Tap: Add controls
4. Close: Control Center by swiping up from the bottom center of the screen.

Swipe down from the top right to open the Control Center, tap the + icon in the top left, in the bottom tap Add a Control, search Orientation Lock, add it back.