I can't get the HDMI display to work with ubuntu-mate-20.04.1-desktop armhf or arm64. No HDMI display but there is HD activity - tried a USB HD & thumb drive?
Tried:
hdmi_group=1
hdmi_mode=1
dtoverlay=vc4-fkms-v3d
Got the Raspberry Pi Imager to download/load Ubuntu Desktop 21.04 (RPI 4/400) without a problem.