can cellular be installed later on an ipad 7th generation

From manufacture, an iPad is either a WiFi Only model - or is a WiFi & Cellular model. It is not possible to change or upgrade any internal hardware elements of a device.

If you have one, but need the other, then you’ll simply have to replace the device with the required model.

If you are ever likely to need cellular capabilities (even if your just want access to GPS features - as these are part of the Cellular chipset), it is always advisable to purchase the a Cellular model at the outset. Similarly, always purchase a model with more RAM than you think you may need - as you can’t add this at a later date. Too much is always better than too little.

I hope you find this information and advice helpful.