Apple WiFi calling not working with PPoE WAN, only double-nat with DMZ

Recently set my parents up with a nice setup including two TPLink EAP 225s, an EdgeRouter X, and a DLink DSL-520B.

One AP mounted in the middle of their old two-storey farmhouse, and another AP in a nearby detached garage.

The whole purpose of these heavy-handed network upgrades for them, was for reliable Apple WiFi Calling to help them phase out their home phone service.

With the provider modem, everything works with double-nat and DMZ forwarding all ports. Wifi calling can be a bit flaky and reluctant to register, but it doesn’t work AT ALL with PPoE for the wan connection on the EdgeRouter X.

I’ve tried mucking about with the settings for IPSEC passthrough (figuring some old settings were hanging onto the old WAN interface), but no luck.

Anyone have any ideas? Would be nice if apple provided some diagnostic capability in iOS for wifi calling 🙄