Friedrich,
So I just tried the command "signtool verify MYINSTALLER.exe" and it returns:

"SignTool Error: A certificate chain processed, but terminated in a root
certificate which is not trusted by the trust provider."

I know it is probably a question for Sectigo, but perhaps as a re-seller partner you could answer: Is it normal that the Sectigo code-signing certificates are not trusted or have I made a mistake with purchasing / installing my certificate?
--Rich F.