If an automated security scanner cannot scan your external application, you can submit a full penetration test (pentest) report. The pentest should be recent (ideally within 30 days of submission) and must include all actionable vulnerabilities that have been remediated.
Alternatively, you can use Dynamic Application Security Test (DAST) tools such as OWASP ZAP, Burp Suite, VeraCode Dynamic Analysis, Intruder, or Acunetix. The DAST scan report should provide details about the current security risk level and the likelihood of risk exploitation for each request and response payload.
If security scanning tools refuse to run on external endpoints, you can use a Dynamic Application Security Test (DAST) scanner like ZAP, Burp Suite, HCL AppScan, or WebInspect. These tools are designed to scan external endpoints and identify vulnerabilities. Be sure to include the reports from these scans in your security review submission.