When you say it wont even click in, do you mean it wont reset or it wont test?
Are you aware of the required regulation concerning installing CUs?
u should not be changing your own cu, it not done. anyway sounds like a crossed or borrowed neutral
Just to add: this is not safe DIY work for several reasons never mind legal issues, how was the supply made safe during changeover?
Now you have done it you find another good reason not to try DIY on such a job.
As suggested most likely problem is circuit neutrals on wrong neutral bar. all circuits must be on one side or the other, you can't have a live on sw. and N on rcd half and vice-versa, If this is not obvious to you then I think you are beyond your knowledge level and should seek qualified help,
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