Why can I get internet on my iPhone while traveling in my vehicle but not only new IPad Air

Your iPhone clearly has access to a Cellular network - with an active Cellular Service plan.

Your iPad, by contrast, may not be a WiFi+Cellular model - and might be the cheaper WiFi Only model. If the latter, you’ll not be able to directly connect to Cellular. If your iPad is a Cellular model, have you arranged a Cellular Service plan - and/or installed an active SIM card?

If you can provide more information about your iPad, such as its model number, we may be able to offer suitable guidance.

Do you have an iPad that is wi-fi and cellular? If you do not have cellular or the iPad is not connected to your iPhone's hotspot, it won't get Internet. One other possibility, does your vehicle have its own wi-fi hotspot for you to connect to?