Monitor recognised by kernel but not X11

I have three monitors - one via HDMI from an Asus Z370-P motherboard (Intel UHD Graphics 630 driver: i915) and two via DVI from a Radeon HD 4650 (AMD RV730 PRO [Radeon HD 4650] driver: radeon).

On boot, the HDMI shows the login screen, the other two are “CL grey”.

On login, the HDMI monitor dies and the other two run the X session as you would expect.

lshw shows the kernel sees the i915 (I think that’s true?):

# lshw -c video
  *-display                 
       description: Display controller
       product: UHD Graphics 630 (Desktop)
       vendor: Intel Corporation
       physical id: 2
       bus info: pci@0000:00:02.0
       version: 00
       width: 64 bits
       clock: 33MHz
       capabilities: pciexpress msi pm bus_master cap_list
       configuration: driver=i915 latency=0
       resources: irq:127 memory:f6000000-f6ffffff memory:d0000000-dfffffff ioport:f000(size=64)
  *-display
       description: VGA compatible controller
       product: RV730 PRO [Radeon HD 4650]
       vendor: Advanced Micro Devices, Inc. [AMD/ATI]
       physical id: 0
       bus info: pci@0000:02:00.0
       version: 00
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi vga_controller bus_master cap_list rom
       configuration: driver=radeon latency=0
       resources: irq:139 memory:e0000000-efffffff memory:f7220000-f722ffff ioport:e000(size=256) memory:c0000-dffff

but xrandr does not see it…

Screen 0: minimum 320 x 200, current 3840 x 1080, maximum 8192 x 8192
DVI-0 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 478mm x 269mm
   1920x1080     60.00*+
...
DIN disconnected (normal left inverted right x axis y axis)
DVI-1 connected 1920x1080+1920+0 (normal left inverted right x axis y axis) 477mm x 268mm
   1920x1080     60.00*+
...

What must I do to have CL recognise the i915 HDMI monitor, please?

Many thanks,
Jerry

Edit: What must I do to have X11 recognise the monitor, please?