It does sound like the chat has let you down, I’m sorry to hear that. You could certainly try the number, but as it’s now out of office hours, I don’t know how successful that will be.
You could try emailing email@example.com and seeing if they reply to you before tomorrow morning (and if they don’t, try calling then).
Alternatively, it may be a Monzo staff member such as @kieranmch, @BethS, @cookywook or @simonb will see this post at some point before tomorrow and either get in touch with you directly or be able to advise you more accurately on what course of action you should take.
Hope that’s of some help!