Hi @Miles M ,
It seems you have already installed the Exchange server successfully.
If you are looking for how to install Exchange server in child domain, here’s a similar thread for you reference. Although it is Exchange server 2013, but it’s same to Exchange server 2019.
How to Install Exchange Server in Child Domain Server (microsoft.com)
Is there any other exchange server in your environment?
According to the error message” Server Error in '/ecp' Application”, The issue may occur with the Oauth certificate is missing from the Exchange server.
I found it’s documented in below link. Please reference it and be free to feed back.
You can't access OWA or ECP after you install Exchange Server 2016 CU6 (microsoft.com)
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