Not quite sure I understand the question,but to achieve the workflow you linked to, each environment would already be configured; you'd have a fully working Enterprise deployment in environment A and a fully working Enterprise deployment in environment B, both using the same front-end URLs. All traffic is getting routed to the "primary" environment and you're replicating data using the DR tool to the standby environment.
So that being said, your new portal should already be talking to your new server. The blog below goes into a bit on URL resolution when setting up secondary environments for migration, but it's applicable to DR as well:
Migrate to a new machine in ArcGIS Enterprise using the WebGIS DR tool