Unable to pair iPhone and iPad after using Airplane mode

Whilst it is technically possible to do so, be aware that Bluetooth pairing between an iPad and iPhone is generally pointless. If your intention is to use Cellular Hotspot features of your iPhone, this link is optimally achieved using the native WiFi hotspot connection - this being a largely automated process. Hotspot connections over WiFi offer substantially greater throughput, range and reliability.

How to set up a Personal Hotspot on your iPhone or iPad – Apple Support

If you still wish to pair your iPad and iPhone over Bluetooth, you first need to manually enable the hotspot feature on your iPhone:

Settings > Mobile Data > Personal Hotspot > Allow Others to Join - set to ON

Next, Bluetooth for both the iPad and iPhone must be placed in Discovery mode:

Settings > Bluetooth (accessing this page will automatically place the device in Discovery mode)

Whilst both iPad and iPhone are in Discovery mode, you can mutually pair the devices.

Again, I must stress, unless you have an unusual “edge case”, pairing of iPad and iPhone over Bluetooth has no benefit.

How are you trying to pair them? Why do you want them paired?

You mean how do you do something on one and it transfers over to the other device so you can work on it from there. Both devices have to be on the same Apple ID.