Incorrect hostname with private networking

I've set mongo-2.railway.internal (or mongo-2) as the endpoint name for private networking on the UI, however, upon running hostname returns railway

Service ID: 46c01986-5e99-4fd6-a662-f79de7b4dd67
Deployment ID: 1732be1e-b0c3-443f-a7cc-de8808e6cf43

totally normal, you want to read from RAILWAY_PRIVATE_DOMAIN

mongo relies on hostname to initiate replica setup.

ah okay you didn't say it needed to be HOSTNAME
just set a variable HOSTNAME=${{RAILWAY_PRIVATE_DOMAIN}}
https://utilities.up.railway.app/env-vars?prefix=HOST