You can configure the SNAT server so that the ECS without an EIP bound can access the Internet.
For details, see section "Configuring a SNAT Server" in Virtual Private Cloud User Guide.
If a large number of concurrent connections are required, you are advised to use the NAT Gateway service provided by the public cloud platform.
The NAT Gateway service offers the NAT function for ECSs in a VPC, allowing these ECSs to access the Internet using an EIP. For more information about NAT Gateway, see NAT Gateway User Guide.