If you select Export As.. and choose TIF + Largest Available Dimensions, this should give you a full res image in the TIFF format. I just tried this myself with a HEIC format image I captured on my iPhone 12, and the resulting image is 36.6 MB TIFF image at 4032 × 3024 pixels.
If you are not seeing this, let me ask... What happens when you try Export As and choose Original? How large is this image? If it is also 2048 px on the long edge, then I might guess you somehow are working with 2048 px copies of your images in Lightroom.
One way this could happen: did you originally import the images via Lightroom Classic on the desktop? LR Classic syncs only 2048px "smart previews" to the cloud, and this is all that LR on your iPad would be able to export.
If Lightroom Classic is not part of the equation here, could you still verify the original image dimensions in LR iPad, by selecting an image and change from Edit view to Info view when viewing that image full screen? Can you confirm the pixel dimension of the image are great than 2048px?