Dual Boot Ubuntu 18.04 and Phoenix OS

[sabusalab]

I did a dual boot on Ubuntu 18.04 and the other OS is Phoenix. I installed the Grub Customizer. When I reboot, Grub did not show up. So I made a configuration on Grub Customizer. I choose under default entry>predefined>Phoenix OS. I rebooted again, no Grub again and booted directly in Phoenix OS. I can not boot my Ubuntu anymore. How can I configure my Grub Customizer since I installed it on Ubuntu? TIA....

[oldfred]

Do not know grub customizer.

Are both systems installed in same boot mode? or both UEFI or both BIOS?

May be best to see details, use ppa version with your live installer or any working install, not older Boot-Repair ISO:
Post the link to the Create BootInfo summary report. Is part of Boot-Repair:
https://help.ubuntu.com/community/Boot-Info and:
https://sourceforge.net/p/boot-repair/home/Home/