It is not possible to deploy Discovery Nodes on an untrusted domain, in order to run discoveries for untrusted domain resources, local discovery nodes must be used with proper credentials to access the untrusted domain by selecting "Use alternate credentials" when creating the discovery.
1. Open Configuration Manager, head to Discovery Management | Manage Discovery Clusters and create a cluster for the untrusted domain.
2. Add Discovery Nodes from the same domain your ER server is on and make sure they are enabled.
3. Head to Discovery Management | Manage Discoveries and create a new Discovery, assign it to the Cluster created on step 2. Make sure to select Use alternate credentials, click the "..." button, click on the add button to enter the alternate credentials from the untrusted domain.
4. Click "Next" to configure the discovery scope, according to the credential provided, the untrusted domain should appear on the list.
5. Configure scheduling options.
