Enabling admin connection on SQL Server Express to fix logon trigger
The documentation for "Diagnostic Connection for Database Administrators" states:


  
  To conserve resources, SQL Server Express does not listen on the DAC port unless started with a trace flag 7806.
  


That TF needs to be specified either on the command line, if you are starting SQL Server via the command line, or in the Registry if you are starting via the Services interface (the typical method).

To get this into the Registry:


open a command prompt
run regedit
navigate to the following key:  


  Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL{version_number}.{instance_name}\MSSQLServer\Parameters


where {version_number} is the 2-digit number of the release, such as "11" for SQL Server 2012, and "14" for SQL Server 2017.
Add the following String Value:

SQLArg3
Edit the new String Value to provide the following value data:

-T7806

Please note: upper-case "T", and no space between the "-T" and the number!