Hello, I am unable to download the IOSupdate 15 and unable to use my cellular

First concentrate on fixing the cellular. No service or searching

Go through the various suggestions in the support article below but if they don't help, and your network carrier can't help, then  contact Apple Support for further investigation.

Support article:

https://support.apple.com/en-us/HT201415

or: contact Apple Support 

Once SIM problems sorted out you can concentrate on updating.

Check 

• you are on a good Wi-Fi, 
• on charge with more than 50% charged, 
• no VPN installed 
• low power mode turned off. 
• Fully working network connection via SIM.
• Check Settings, General to make sure you have no Beta profile installed.

Resources:-

Update phone

Update your iPhone, iPad, or iPod touch - Apple Support

Phone will not update

If your iPhone, iPad or iPod touch won’t update - Apple Support