While installing the NMAS SAML method in an external user store, DN is added as Provider ID instead of the metadata URL.
To resolve this issue, perform the following steps:
Log in to the Administration Console which has the external user store.
Go to Roles and Tasks > NMAS > NMAS Login Methods > SAML Assertion > Affiliates.
Select the respective Affiliate and change the provider ID to the identity provider metadata URL. For example, https://www.trunk2.com:8443/nidp/idff/metadata.