All you have to do is, try resetting password outside SailPoint as you have tried, this should trigger REST API/Workflow in SailPoint to further cascade. I did a POC last year and you can check logs in your DC to make sure AD Interceptor and IIQ are connected.
I am assuming you have installed and configured the PWI on one of the AD Domains. In IIQ, there is an OOTB Password Intercept that intercept the password change in IIQ and depending upon you configuration, it updates the password on target applications. You should be seeing this workflow triggering on password change and also you can enable the logging for PWI to intercept and validate your change.