I posted last year in Nov/Dec on this thread about my June/July 2020 Unwanted Imports HM Aeron mesh failure and my experience contacting them to get the replacement part. Just go and dig for my posts back then.
The link to contact them is
https://www.unwantedimports.com/general-warranty-support/
which can also be found on the Contact Us page on their website.
Unfortunately, I had to pay for the part as per their specified cost + shipping via air (faster) and they did not reply to my email several months later on whether the warranty claim was successful or not. Not sure if they are still very responsive now since they are based in the UK.
The fact that your chair was bought in Sep 2020 is also making me wonder if there was some issue with HM manufacturing....