If Wi-Fi is not working and I want to do iCloud backup through cellular data

Only if you have an iPhone 12 or 13 or SE 3rd gen with a 5G connection you can use over the 5G network.

Try to get the Wi-Fi network working, see the following Apple articles:

Connect to Wi-Fi on your iPhone, iPad, or iPod touch

Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network - Apple Support

Recommended settings for Wi-Fi routers and access points

Recommended settings for Wi-Fi routers and access points - Apple Support

If you need help with your Wi‑Fi password

If you need help with your Wi‑Fi password - Apple Support

If they are On, turn Off Hotspot & VPN.

Then see -> How to back up your iPhone, iPad, and iPod touch with iCloud

How to back up your iPhone, iPad, and iPod touch with iCloud - Apple Support