Re :Celina
The following are the basic requirements for applicability of AC21 rulesin your situation.
You should have an approved I-140
Your I-485 should be pending for 180 or more days
You should find a similar job with a new employer.
In your case AC21 rule is not effective as according to you, you have not completed 180 days after you have filed your I-485.
What you can now do is wait for the required 180 to get over. If your AOS is not adjudged before that, you can give a letter from your new employer with job discription as given in your LC. Also ensure that your old employer doesn't cancel your I-140.
I am not an attorney and it is not a legal advise. Please consult your attorney for possible remidies in your case.
Hope this helps !