As mentioned the processing and issue of Residence Card is taking anything up to 6 months.
It's such a pity that the COA is not well understood. That document is issued by the Home office as your proof of status in UK and is meant to indicate that you, as the holder, should be treated as if you actually had a Residence Card.
If you have an offer of employment but are being refused National Insurance number you should contact your MP and explain. Your MP may be able to get things moving for you.