Hi Mark,
Since your endpoint works as stand alone (IP dialing method), under H323 settings Call Setup Mode set is to Direct which is correct. Erase all the parameters configured under H323 and don't forget to click Save button to apply the changes.
See my sample configuration of one of my deployed endpoint.

Also under SIP settings, leave all parameters as blank.

Then go to Diagnostics > Troubleshooting and Re-run Diagnostics.
You should not get any error but only a warning like this:

This WARNING tells that the system is not registered to a H323 Gatekeeper. It means the system cannot make H323 calls using extension or E.164 alias. In your case IP dialing method and will work fine.
Regards,
Acevirgil