I am saying cellular issues while trying to use cellular network please resolve this issue or solve iOS update

1.

Try to remove and reinsert the SIM card.

If it doesn't help visit your carrier, maybe the SIM card is damaged, ask for a replaced (new) card. Normally it is free.

Ask also to check if the SIM tray is bent, as even a tiny misalignment can cause the SIM card to not fully connect with the internal contacts in your iPhone.

2.

See these Apple articles below:

If you see No Service, Searching, or SOS on your iPhone or iPad

If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support

Manually update your carrier settings on your iPhone or iPad

Manually update your carrier settings on your iPhone or iPad - Apple Support

Update your network provider settings manually on your iPhone or iPad

Update your network provider settings manually on your iPhone or iPad – Apple Support (UK)

If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart

If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart - Apple Support

How to unlock your iPhone for use with a different carrier

How to unlock your iPhone for use with a different carrier - Apple Support

• Apple can't unlock your iPhone for use with a different carrier. Only your carrier can unlock your iPhone.

You need to contact your Carrier.

Or do you need the PUK code?, if so see this Apple article: Use a SIM PIN for your iPhone or iPad

Use a SIM PIN for your iPhone or iPad - Apple Support