In most cases, 5G activates automatically if you're using a 5G-enabled device within a 5G coverage area.
If needed, you can manually enable it by:
• Going to Settings
• Selecting Mobile Networks
• Choosing your SIM
• Tapping Preferred Network Type
• Selecting 5G
You can also refer to PTA’s guide: https://x.com/PTAofficialpk/status/2036474155122696475