In accordance with the last paragraph in their Notice of Rejection which refers to 'the instructions below' and a PIN.
You didn't post these instructions so we're none the wiser.
Get on to the London Tribunals website NOW and try to register an appeal:
Do it NOW.
Grounds:
Procedural impropriety
Contravention did not occur
You rely on your formal representations.
You apologise for submitting your appeal outside the 28-day period which was due to (something plausible please and true) and would hope that given the strength of your case and prospects of a successful appeal the adjudicator would none the less register your appeal.