I currently run a headless RPI4 in a custom enclosure (the RPI4 isn't easily removable) that doesn't have access to the HDMI display ports. Generally, to install an OS for the RPI4, I've always made an empty ssh file in the /boot directory of the SD card after extracting the new install (this works for the Ubuntu Server images as well). However, this doesn't work with the ubuntu-mate-20.04.1-beta2-desktop-arm64+raspi.img since the SSH server is not installed by default. This would be easy to do with APT after the initial provisioning as the notes indicate, however, I'm stuck without access to the HDMI display outputs.
My main question, is there a way enable SSH or some other form of remote management of the Ubuntu-Mate OS that I can configure or enable prior to deployment?
Also, in an effort to try something besides SSH, I connected to the RPI4 UART serial console PINs via an FTDI USB adapter & PUTTY, but I'm not able to login since a login prompt never appears (it remains at Started WPA supplicant), however, I do see the boot debug and output. Trying again with the same setup but instead with a fresh Ubuntu Server ubuntu-20.04.1-preinstalled-server-arm64+raspi.img install allows me to see the boot output as well as login. I've confirmed that the cmdline.txt, config.txt and usercfg.txt files are identical between the Ubuntu Server and Ubuntu-Mate inital installion install files.
Screenshot of the serial console with Ubuntu Server is on the left, Ubuntu Mate Beta2 on the right: