A partner must allowlist fixed IP addresses for your service, which speaks a custom TCP protocol at very high connection rates. Which load balancer fits?

A Network Load Balancer (NLB) fits. It works at layer 4, so it handles raw TCP and UDP, not only HTTP. It handles very high throughput and connection rates. It can have one static Elastic IP per AZ (Availability Zone), so the partner can allowlist fixed addresses.

"Application Load Balancer" is the tempting wrong answer. It only speaks HTTP and HTTPS, and its IP addresses change over time. It suits path routing, not fixed-IP TCP services.