I have opened a Pages document in Pages v14.2 on iPadOS 18.0.1. It made no difference whether I initially clicked the edit button on the toolbar. One should be able to click ⋯⃝ and choose Settings > Auto-Correction > Check Spelling and slide it on or off. However, in my case, Check Spelling is grayed out and off. Yours may be on and can be slid off to disable the red underlining of spell-checking.
Apple's iPadOS Pages documentation makes it sound so easy to do this, but not on my M4 iPad Pro with Pages v14.2.