Hey @Oliver! Stepping in here to try to help.
What error are you running into with your Postgres connection? Which IPs have you whitelisted?
List of IP addresses to whitelist here: https://docs.retool.com/docs/network-storage-requirements
And also here (should be the same addresses for on prem instances): Changes to Retool’s IP addresses for cloud and on-premise connections - #8 by Kabirdas