Cannot reinstall macOS after latest update
This is a complicated situation:
iMac 2010 with ATI graphics, 500 GB original HDD, DVD failed after few years and I've replaced it with 275 GB SSD. Installed fresh OSX to 275 GB which is a primary drive. All working as normal. OSX was updated until High Sierra I think (10.13.xx); I'm not an expert with the numbers or OS names.
Due to iMac age, it wouldn't allow installation of El Capitan or any latest OS - no problem for me. However, the OS insisted on some security updates and then it wouldn't boot anymore.
I booted into recovery mode to reinstall the macOS. It checked if I'm eligible and confirmed it's OK. Then it started to download the files from their server... and it stopped. As I understand the log file, it fails with some "El Capitan" messages. I suspect it doesn't know I could only get the High Sierra. No luck to make the system work!
Took Ubuntu 18.04.2 LTS and installed it onto 275 GB. I had to edit the GRUB to make the graphics work. Installation was quick and easy. However, after the reboot, I get no graphics at all (I think Ubuntu starts but due to driver incompatibility it shows only a black screen). I had to connect a USB keyboard and managed to get into Ubuntu's boot options. Here, the keyboard doesn't work anymore. So I'm stuck.
After a (hard) reboot, iMac always boots into Ubuntu without giving me an option to select which OS to boot in. The original bluetooth keyboard is not responding at all so I cannot get up to the boot drive (holding the Option key). Command+R also doesn't work.
Is there any other way than removing the 275 GB drive?
iMac 21.5", macOS 10.13