Epicor URLs are specific to each instance and they have the below form
https://<host>/<instance>/api/<version>/<ServiceOrEndPoint>/<method>
I am more concerned/impressed on how you wrote an integration without access to an API. The end points are VERY specific to each business object so just having a database won’t be very useful.
That is you can’t hit the below URL to get Customers
https://<host>/<instance>/api/<version>/Customer/
Epicor uses oData standard for all their Services and there are a lot more integration points that use RPC so having just a DB wouldn’t give you enough information to write any kind of integration IMO