DWP complaints, benefit cap grace period

Problem:
I recently queried, through my Universal Credit journal, why I was not granted the benefit cap grace period when I first claimed Universal Credit in September 2023.
I checked their guidance - all good here, I met the requirements, but…
I informed my work coach at the time that I was having problems receiving my final pay, ACAS became involved, and the employer eventually paid me in instalments.
Initially, I was told that the benefit cap had been applied automatically by the system.

Later, I was told that I did not qualify for the grace period because my August 2023 earnings were below the earnings threshold. I was asked to provide evidence, so I submitted my final payslip.

I also contacted HMRC, who confirmed that my August 2023 earnings are recorded correctly. The complication is that my final payslip was originally issued later because of the insolvency, although HMRC appears to have corrected the earnings to the appropriate period.

Despite repeatedly asking, the DWP answer is - system said 'no'.
The case manager has now said that they are unsure what I am asking and has refused to discuss the matter further or refer my query to someone else.

What are my options?