Cannon boot into Ubuntu after dual boot

My main OS is Windows 8. I have replaced my optical drive with the second SSD drive and installed Ubuntu 18.04 there. However, after the installation process was done and my laptop restarted – I cannot boot into Ubuntu anymore. I have set the second SSD drive at the first place in the boot order in BIOS, but it seems like it’s skipping it and automatically booting into Windows. I’ve disabled both secure boot and fastboot options. Do you have any suggestions what to do?