I have tried literally everything and can't get a terminal to display anything and looking at the display I don't see any 888 errors so I'm not sure what to try next.
Don't give up hope. I think the fun in these vintage machines is the *journey* and *challenge* of getting them running, more than the end result itself, unless it's a production machine you agreed to help somebody with...
If you look earlier in this thread you'll see I struggled mightily with getting mine to boot from CD-ROM. It must have a CD-ROM that supports 512-byte sectors, and even then not guaranteed to work, so try a different CD-ROM drive if you have access. Some of the older drives have a jumper to select 512 vs 2048 byte sectors. Also, the earlier RS/6000s seemed particularly sensitive to SCSI bus termination, so is your external drive terminated properly?
I had another RS/6000 barn find that I had trouble getting serial access, and it turned out it was configured for 19200 baud, which I would have expected to result in garbled characters, but it was just blank. Might try some different baud rates.
...and for sure make the cable
@Radix suggested, they're smart guys, follow their advice. I can tell you that my 220 works with a normal crossover cable connected through a serial-usb adapter to a windows 11 machine using putty, but you're obviously isn't.