Power Saving Mode or Low Power Mode may disable 5G cellular service .

To enable 5G on your iPhone, follow the simple steps below:

1. Go to "Settings”

2. Select “Mobile Data”

3. Tap on “Mobile Data Options”

4. Click on “Voice & Data”

5. Select “5G On”

6. Set 5G Standalone "On"

If you don't see 5G in the status bar

1. Make sure that you’re in an area with 5G coverage.

2. Go to Settings > Mobile Data > Mobile Data Options. If you see this screen, your device has 5G activated.
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3. Turn on Airplane Mode, then turn it off.