Can’t boot into ubuntu in dual boot windows 10 and ubuntu 18.04

I am using a Dell laptop inspiron 15 3000 with operating system windows 10. I have installed ubuntu 18.04 in part of my internal hard disk after installation when I boot my computer I can only log in to windows 10 but when I press F2 and go to the bios boot menu and change the boot sequence to legacy not UEFI from the boot list option and save it I log in to ubuntu only not windows and the GRBU boot loader window never appear to choose between the 2 operating systems