iCloud Lock

I have been given my mums old ipad mini to use at the gym (I don't want to risk my new ipad!!).

The story is this.........

My Mum was using it on her old apple id, she didn't realise you could change the email address so she set up a new one when she replaced the ipad mini with a newer model. The email used with the old apple id is now defunct as her internet provider discontinued the account when they changed internet providers (we cant sign in to that at all, it no longer exists). She doesn't remember the password on the apple id.

I want to rid the ipad of all the data and use it with my apple id.

Things I have attempted....

tried getting a verification code sent to her iphone (which matches the number she was using for the old apple id) but nothing comes through (the iphone is now set up with her new apple id).

Restoring ipad through itunes by pressing 'restore ipad' this doesn't work as find my iphone is switched on and I cant turn it off without the apple id password.

Connecting ipad to itunes and doing a hard reset, then restoring- tried this 3 times and each time it gets to near the end and then says "ipad couldn't be restored as cant be found". despite the fact it is plugged in and charging and acknowledged in itunes. Tried all 3 usb ports on windows laptop.

I should add that i know the passcode and that's fine, I can access all the functions apart from anything that requires the apple id password.

Is there anything else I can try?

Thanks.