If you are using o2 travel as your data connection on phone it's most likely because the speed is too low.
A tethered connection will run at half the data speed of the primary devices connection which we know from experience of o2 travel is never better than around 0.5mbs.
The iPad won't see any data at those kinds of speeds and throw an error.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)