headless pi not accessible

I would like to use a RISC OS pi4 remotely via VNC.
The pi runs VNC Server at boot and I can access it from !Avalanche on an ARMX6 provided there is a monitor attached to the pi..
However if I unplug the monitor from the pi and reboot to use it headless it is not accessible.

It is hard to know what is going on because there is no visible output.

The config.txt contains the following lines.

disable_overscan=1
fake_vsync_isr=1
framebuffer_swap=0
gpu_mem=64
init_emmc_clock=100000000
ramfsfile=CMOS
ramfsaddr=0x508000
kernel=RISCOS.IMG
[pi4]
enable_gic=1

The pi is a 4Te running 5.28

hdmi_force_hotplug=1 in config/txt should allow headless working.

John as you have a 4te, you will I suspect have the !4teTools app on your computer, within that is the Display section and in that is an option to run the Pi (4te) headless with no monitor connected.

Thanks David and Chris
Using 4teTools has done the trick. Among other things it changed the config.txt to include “hdmi_force_hotplug=1”

here is the new config.txt:

hdmi_force_hotplug=1
hdmi_group=2
hdmi_mode=84
disable_overscan=1
fake_vsync_isr=1
framebuffer_swap=0
gpu_mem=64
init_emmc_clock=100000000
ramfsfile=CMOS
ramfsaddr=0x508000
sdtv_mode=2
kernel=RISCOS.IMG
max_framebuffer_width=3840
max_framebuffer_height=2160
hdmi_pixel_freq_limit=250000000
#
[pi4]
enable_gic=1
hdmi_enable_4kp60=0