any ideas/hints to help me move forward please?

It looks like your app is failing on TLS handshake because the Erlang runtime doesn’t trust the certificate authority (CA) it’s seeing.

What could be happening:

By default, Phoenix/Erlang apps need access to a system CA bundle to validate HTTPS connections (ex: when your app connects to external APIs or Postgres over SSL).

On Railway, if the CA certificates package isn’t present in your environment, Erlang/OTP can’t find the trusted root certificates > handshake fails with Unknown CA.

I am going to assume you're using Nixpacks, so on Railway I would add cacert to your build. Update your nixpacks.toml like this:

[phases.setup]
nixPkgs = ["erlang", "elixir", "cacert"]

Also, make sure the config name is "Phases" (plural) and not "Phase". I had seen this issue in a past thread.
This installs the trusted root certificates so Erlang can complete the TLS handshake. After redeploying, the Unknown CA error should disappear.

Thank you for your help and sorry for the delay in getting back. I was checking this thread for a few days but then stopped checking and I don't get any notifications.

I updated the nixpacks.toml by adding the phases.setup there. and then ran 'railway up' and got this error:
error: Unable to build profile. There is a conflict for the following files:

/nix/store/y2zh06pccwcvz43xwgd8mr8pbqflkqww-nss-cacert-3.107/etc/ssl/certs/ca-bundle.crt

/nix/store/zw290ir9xdzlhf385ywm3a5ss08d0fgp-c5702bd28cbde41a191a9c2a00501f18941efbd0-env/etc/ssl/certs/ca-bundle.crt

error: builder for '/nix/store/r77h2h222hmj1annpir4099pzvwvc5ch-user-environment.drv' failed with exit code 1

Also I did not understand what you meant by "make sure the config name is "Phases" (plural) and not "Phase"." I just copied and pasted the exact contents into the toml and redeployed.