Masonry chimney cleanout replacement

Hi Good People,
You've done such a great job that now I'm back for looking for more advice.
The steel cleanout door (and its frame) at the base of my interior, brick-faced masonry chimney is not cemented/mortared in place. This is a condo that I just bought and the inspector missed this detail.
I don't mind cementing it back in place. But I want it done a) Securely and b) Easily.
Is there a compound that I can get in a caulking tube that will do the trick?
Should I use a special cement, or mortar?
Any web pages that you know of that can show the correct steps?

Thanks in advance,
Bill