Depending on the provider DNS can take a while to update. Cloudflare usually updates very quicky, but others are not as fast. It might take up to 48 hours, though usually it's not that bad.

You can use https://www.nslookup.io/ check whether or not the DNS is resolving to the proper values.

You also may have set the records incorrectly if it's still not working after that time.

If you can give more info like what DNS provider you're using, or which domain / which records Railway told you to set I could see anything looks off.

Your DNS records look correct now, but the certificate process didn't detect them in time. To restart certificate issuance:

1. Remove the custom domain from your service in the Railway dashboard
2. Re-add the domain
3. Set up the CNAME and TXT records shown in the dashboard with your DNS provider

Your certificate should provision shortly after that.