Dual SIM: Cellular data on one, WiFi calling over cellular on second while traveling

There was a thread a few months back from Calif2Paris (Dual SIM: Setting Wifi calling on one, Wi… - Apple Community) that seemed to be exactly what I wanted to do, namely:

• Primary SIM at home is set up for WiFi calling
• Secondary SIM for international travel gets data and can receive calls/texts
• Primary SIM while traveling should use WiFi calling over cellular (is this IMS)

One of the replies in the thread from AppleGeniusSupport seemed to indicate that this should be possible, specifically with AT&T, by disabling "International Day Pass" and "Pay Per Use" in the AT&T account and disabling "Data Roaming" under Cellular options for this line on the iPhone.

I did try this while I was traveling; however, I could not get it to work. Even with both options disabled in my AT&T account, the AT&T line would still connect to the international cellular provider and would not switch over to WiFI calling over cellular because it thought it had service.

One thing that I tried to do while I was traveling was to select one of the providers instead of using "Automatic" under Options > Cellular > Network Selection. When I did this and traveled to another country, I did see that WiFi calling over cellular was enabled, but it was for the previous provider that I selected, not AT&T.

This seems to imply that I would need to select AT&T while at home so that when I traveled, it would switch to WiFi calling over cellular since the line would not be able to connect to AT&T. Is that correct? If so, is there a way to force this option to be available to me? When I am at home in the US, I do not have the Network Selection option on my AT&T line, so I'm wondering how I might be able to accomplish this.

Or am I missing something else in either my iPhone or account settings?