GTM probe from non self IP

I have a GTM on private self IP addresses, I do however have public DNS listeners. This is working fine for devices within our AS. However, I wish to monitor an external generic host from these devices but the GTM sends the probe with the private source IP address which are not routed off our AS. How can I make the GTM probe external hosts with a public/different source IP address? Can you change the probe source IP address within GTM?

Thanks.