In most countries/regions, Apple offers an unconditional 14-day returns policy - where an unwanted (undamaged) device can be returned for exchange or refund. Upon expiry of the 14-day returns period, this opportunity is lost.
As you are clearly well beyond the 14-day returns period, if you wish to replace the iPad with a WiFi+Cellular model, your only option is to sell your iPad privately - and put the proceeds of the sale towards the cost of a replacement iPad. Be aware that as a second-hand device, used or otherwise, you are unlikely to recoup anything close to your original outlay.
However, all is not lost. If you simply wish to benefit from the ability to access an internet connection when away from WiFi, you have two options...
1) If you have a mobile phone (such as an iPhone) with an active Cellular plan that supports tethering, you can use the phone's hotspot function to share the phone's internet connection with your iPad:
How to set up a Personal Hotspot on your iPhone or iPad - Apple Support
Share your internet connection from your iPhone - Apple Support
Connect iPad to the internet - Apple Support
2) If you don't have a suitable mobile phone, you might instead consider purchasing a Cellular Router accessory - often known as MiFi. These are available from most Cellular Operators and good computer stores.
Be aware that while these options can provide easy and cost effective access to the internet when away from WiFi, your WiFi Only iPad will not benefit from GNSS/GPS positioning capabilities. While not reliant upon Cellular features, GNSS functions are integral to the iPad's Cellular chipset.