SILU DUNGJAN TAMANG

Hi there, I am currently on an emergency tax code which needs fixing ASAP. I can log-in to Government Gateway with no problem, however when I enter my number, date of birth and NI number I keep getting a "we cannot confirm your identity" message. I have called up the HMRC helpline and they weren't able to verify me either, which is strange as my name, birth date and NI number have never changed in my life so I can't understand how it could possibly be wrong. I don't know where to go from here, can anyone please give me advice on who can help me? Thanks!