Why is it compulsory to update it on wifi only

Find some wifi or take it to Apple Store.

Currently top end 5G iPhones will allow use of cellular to update.

The reasons are to protect the providers' data traffic, but you will find as time goes by the opportunity may come your way.

Not yet.

Ask a friend, acquaintance or family to use their computer, or at a public library.

Alternately, visit your local Apple Store or Apple Authorized Service Provider and they should be able to help you out.

You will first have to make an appointment due to the pandemic.

Call Apple in your region see: Contact Apple for support and service

Contact Apple for support and service - Apple Support

Not the best solution, but probably the quickest........Take the phone down to the coffee place and log on to the WiFi.