Kia ora Craig
Can you please confirm that LTI Advantage is supported in Totara 13 onwards.
I tried to configure an external tool in a Totara 17.6 site using all the URLs from a Moodle 4.0 LTI Provider (LTI Advantage) but none worked.
The URL from an LTI 1.1/1.2 enrolment method in a Moodle course however worked.