FYI:
Today, I was informed by ChiefGyk3D (Mastodon / Twitch) about Efani, a US-based security-focused mobile service that claims to protect against SIM-swap attacks.
“Why Purism Librem 5 Is The Ideal Phone Choice in Terms of Security” - Haseeb Awan, Efani
ChiefGyk3D said at 3:14:00 in today’s stream that Efani uses an AT&T or Verizon (which is known to not support Librem 5) backhaul network.
Assuming this all works on the Librem 5, you should have the option of either T-Mobile and AT&T networks.