IPad pro cellular activation issue

I bought a used iPad Pro cellular & WiFi and it’s all fine apart from not being able to use it with my Voxi (Vodafone sim). Whenever I put the sim in, it starts some sort of activation and eventually gives an error:

I have to take the sim out if I want to continue to use the iPad.

The iPad used to be on EE but I called them and they say that it’s now unlocked.

I’ve called Apple and they say that there’s nothing wrong with the iPad - it’s the sim that’s not compatible.

I’ve called the Voxi (a subsidiary of Vodafone) and they say that all their sims work on tablets and there’s no ‘special’ sim for iPads.

My wife’s Voxi sim does the same thing

Unfortunately, we don’t have a sim from another network but Apple insist that it’s the specification of the sim that may not be compatible with the iPad.

Just to be sure, I’ve reset network settings but it’s made no difference.

Weird thing is that I have no “Carrier lock” in settings but in Network Provider, I have “iPad 49.0”. I have no idea what this is!

Somebody suggested I mess with resetting Re-provision Subscriber account but I’m a bit nervous about messing with that.

Can anyone please help? The iPad is still under warranty so that’s not an issue but I was hoping that there might be something I could do.