Non NI number holder trying to fill in the form online

I am writing on behalf of my mum who owns properties in the UK and has been told that she has been penalised for not submitting a tax return form. The gov has sent her a letter with her UTR but since she does not live or work in the UK, she does not have a NI number that allows her to fill in the form online. How can I help her in this situation? She has already been fined over £500 for late submission.

Hi Sammy,
As your mum lives abroad, she will not be able to send tax returns online.
She will need to send paper copies of any outstanding tax returns.
She can print the returns from:
How to complete your tax return for Self Assessment
Or request them by contacting us here:
Self Assessment: general enquiries
Thank you.