I have looked at all the links and tried the suggestions and I am still having the problem:
SCCM 2012 R2 SQL 2012 SP1 SSRS installed and configured (separate boxes)
When I try to add the reporting services role, it finds the SCCM database and verifies connectivity to it, the SSRS instance drop down is blank.
Using a domain service accout with local admin rights on both machines and sa rights in SQL.
There is a firewall between the Primary Server and the SQL Server and ports 80, 135, 443, 445, 1433 and the high RPC range.
Does anyone know how it is determining the instance? I have read it is via WMI but I am not sure if it is a query or executing a method. I have verified the svc acct has the correct DCOM privs.