Leave the SLC-5/04's serial port set for the default settings: DF1 Full Duplex protocol, 19200 baud, 8/N/1 framing, CRC error checking.
Set up the Digi One IAP's serial port for the same settings.
In the Red Lion, treat the Digi One IAP as though it was a MicroLogix 1100.
The Digi One IAP will take the EtherNet/IP packets requesting read/writes from the SLC data tables and convert them into DF1 Full Duplex, and do the same with the replies from the SLC.
That ought to do the trick !