Dual booting Linux-Mint with Windows 10, UEFI/Legacy problem

I used this tutorial to install Linux Mint in a Windows OS Dell computer.

I quickly found that in order to get into Linux I had to hit F12 everytime the computer booted, and then, select boot from USB flashdrive under UEFI booting.

Now, I wasn’t aware that this would become a problem, but Windows OS is under legacy booting.

From here, it appears the problem is that Grub was installed onto the flash drive and not onto the internal hard drive. The solution given is

sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair
boot-repair

When I do this last step (boot-repair), I’m getting the error

LegacyWindows detected. Please enable BIOS-compatibility/CSM/Legacy
mode in your UEFI firmware, and use this software from a live-CD (or
live-USB)

So, I tried disabling UEFI booting as shown here, but I’m still getting the same error. Any help would be greatly appreciated, I never intended to spend so much time just getting Linux to run on a Dell…

Could I reinstall Linux and instead of choosing a partition, choose to wipe the disk to install just Linux, then re-partition the drive in Linux and install a Windows OS in UEFI booting mode so that both are in UEFI?