Dual SIM: Setting Wifi calling on one, Wifi and Cellular on the 2nd

There have been multiple threads on this issue: using a dual sim phone abroad with a France plan & an AT&T plan (an alternate solution to paying for international day pass / pay per use). I use my French plan (Free Mobile) for data, calling and text (Europe & US are included) but still want to access texts and calls from my US phone over wi-fi calling. Currently, I have to turn my US phone "off" to not incur day pass or pay per use options. There were suggestions to turn off international day pass and pay per use options. I have turned off the day pass option, but can't find the option to turn off pay per use and AT&T customer service says it's not possible. Any leads here on how to do that successfully? The other option suggested was to force the US line to choose a carrier that doesn't have service so it bypasses the US carrier and uses the french. I have not been able to figure that out either from the previous threads. Any help with either of these options, or other options that have worked in the community would be greatly appreciated! Thank you!