The DB-9 port may be set up for DH+ comm's not RS-232. I remember the first time I hit that bug.
For that you would need the UIC adapter.
If you don't have one forum member
GeniusInTraining sells them.
I think you meant DH485. There are several protocols it could be that can either prevent access by RSLogix or make it tricky...
If it is DH485, DF1 Half Duplex, or DF1 Radio Modem, then you still stand a chance.
If it is set for Modbus RTU, ASCII, or Shutdown, you are probably not getting in through that port. I say probably, because with ASCII mode (user tab), there is a way to programmatically switch modes back to System. This is rarely used, so most likely just a distraction from your trouble, but still among the list of possibilities.
DH485 is probably the most common alternative in general use. In my field (water and wastewater), we see a lot of the other protocols used for radio telemetry purposes.
I have RS232/USB cables from PLCcable.com, and RSLinx Autoconfigure has always worked when the device at the other end is set for DF1 Full Duplex.
The DH485 cable Mark sells is excellent too.
It would be a good test to prove your PC/Cable/driver settings by checking with another DF1 PLC just to make sure you don't have a hardware or software problem.