Hi Simon,
If you have are working in a paid cloud license of OutSystems you can request those IP addresses through a support case. Alternatively, you can request CloudWatch access to your OutSystems infrastructure AWS VPC. With cloud watch you can see your self what the public IP addresses are.
Here is some more information on network requirements.
https://success.outsystems.com/Documentation/11/Setup_and_maintain_your_OutSystems_infrastructure/Setting_Up_OutSystems/OutSystems_network_requirements
Regards,
Daniel