Two IPv4 hosts on the internet cannot ping each other

Hi! I have to machines connected to consumer-grade fiber at major ISPs. One in France on the Orange network, and one in the US on ATT.

Each of them have perfectly fine internet access: they can ping and be pinged by every other machine I have access to.

But, very surprisingly, they cannot ping each other. I also have web servers running on both, and neither can curl a web page from the other.

I suspect something is broken in the overall routing.

How can I diagnose what's going on and who do I report this to?