A customer used 2 different domains for agent communication. If Domain A connectivity fails then, agent must communicate through Domain B. What is the best approach you can suggest to customer to make sure agent communication will not be lost?

Customer can configure load balancing communication for this.
Under Settings-> Agent configuration->Add new profile and under communication Settings → select load balancing configuration.

Map the newly created load balancing profile to end machines by creating a mapping configuration under Settings > Agent Configuration.

Please refer help link on the same Agent Configuration