what kubernetes ip to give vendor to whitelist

have a java app on a Kubernetes cluster on IBM cloud that needs to make an outbound SOAP call to a third party vendor's app. This vendor's servers are protected by an IP whitelist. No matter what I do, I can't seem to find the correct IP for them to whitelist. I know this because even though I can hit google, or yahoo, whenever I try to connect to the vendor's URL, the connection times out.

• I curled https://api.ipify.org and gave the IP to the the vendor. The issue is, when the app is redeployed, the IP changes. This is not a feasible solution for an app that is redeployed multiple times a day.
• I have set up a public Network Node Balancer and given the public IP of the NLB to the vendor. Didn't work.
• I have whitelisted the public IPs of the machines the cluster is running on. No change.

As far as I can tell, the the cluster is behind a NAT, and is connected to a VLAN. How can I find the right IP for them to whitelist?

I'm not familiar with IBM cloud and how they do IP addressing, but if similar to say, Azure or AWS, your outbound IP address could be from multiple IP addresses. In Azure, this is clear on the service you are using by looking at properties. There it will list all the possible outbound IP addresses. I'd assume something similar is on IBM Cloud, or they should be able to provide that info if it isn't.