A. Allocate additional Elastic IP addresses to the NAT gateway.
B. Request that the third-party service provider implement HTTP keepalive.
C. Implement TCP keepalive on the client instances.
D. Create additional NAT gateways and update the private subnet route table to introduce the new NAT gateways.
E. Create additional NAT gateways in the public subnet and split client instances into multiple private subnets, each with a route to a different NAT gateway.