Hello, thank you for the question.
I presume you have made a formal complaint to BT Open Reach?.
If so, and they fail to address the problem and to give you a satisfactory service, you can do two things. The first is to report them to OFCOM, the regulator. You can do this here:
Secondly, you can sue BT for providing a sub-standard service and quote the Consumer Regulations Act 2015. By law, they have to provide a good standard of service, otherwise they owe you a refund. You can write to them with a letter before action to demand a refund within 21 days. In the letter you need to put them on notice that you intend to sue them if they do not either resolve the issue within 21 days or fail to refund you. You can then issue a small claim online at www.moneyclaim.gov.uk.
I hope this helps. If you would like further assistance with any of the above, please do not hesitate to ask. If you can accept the answer and give me a star rating, I can offer unlimited follow up Q&A's at no extra charge.