SERIOs are only supported in the local base.
If you need a remote serial port, you may want to look at a BX-DM1E-M BRX PLC that has no I/O, but write a small program that communicates "locally" with the remote serial slave device (or get a BRX with I/O if you actually need remote I/O also). You can then use RX/WX over Ethernet from your 205 master PLC to interface to the "remote" BRX to read and/or write whatever data the BRX has from the remote serial slave device. The BRX serial port can be point-to-point or multi-drop, in case you had more than 1 remote slave device in that physical area.
If you just need to poll/refresh, this can be done pretty easily with a small number of read and/or write communication boxes in the slave (probably combination of MRX/MWX Modbus Read/Write boxes) and 1 or 2 RX/WX Ethernet boxes in the master.