Struggling to submit the request for the return since the 2nd January 2024 there's no response

On the 7th December 2023 i bought 2 jeans online now they can't fit my husband they are big, since the 2nd of January 2024 i've been struggling to return them the system doesn't want to accept the order number and my details it keeps on indicating that they are incorrect. The 0800 number is not working, can i please be assisted ASAP