Secondary portal URL

Hello! We have a proxy server with OTP to secure users connecting to LAN services from Internet.

Lemoldap portal using as second proxy to authenticate user with LDAP. But LL redirecting users that connecting to inner site to portal.innerdomain.ru, that is not known by External DNS.

Can I set up secondary URL for portal like portal.ru to authenticate external users or I must create secondary portal with something like remote authentication?