Allowing source IPs to be visible behind a BIG-IP

The support engineer is correct. However that assumes that the BigIP will actually act as the default gateway.

You have several choices

• Insert host routes to the BigIP for the clients instead of default gateway
• Use policy routing and only route traffic to the clients FROM tyhe service port via the BigIP
• Actually turn the BigIP into your default gateway

That's assuming you're not picking up the original IP via X-Forwarded-For headers or something that you haven't enabled.