Optiplex GX260: How to gain access to CMOS/BIOS when F2 key does not make SETUP run?

After the CMOS battery is removed/replaced the machine boots up to:

Invalid configuration information - please run SETUP program.

 Brief pause, then:

 Performing automatic IDE configuration...

Primary Master: IDE Disk Drive.

Primary Slave: IDE Disk Drive.

Secondary Master: CD-ROM Reader

 Time-of-day not set - please run SETUP program

 Flashing cursor to left of screen. F2 key produces no result on this screen.

If re-booted and F2 key is struck, then a blank screen appears with flashing cursor in top left corner instead of running SETUP.

I have reset the PSWD/RTCRST jumper [= no change].

I have removed and replaced the (full voltage new) CMOS battery [= no change].

If the machine is switched off, it is necessary to remove and replace the CMOS battery again before it will boot up to the automatic IDE configuration + time-of-day not set msg, otherwise it is just the blank screen with flashing cursor.