pacanda :
My motherboard is an ASUS F1A55-M LX3. I am able to enter into the Bios by pressing Delete during startup. Choosing the Boot Priority button shows me then 3 hdd's, 1 CDROM drive (which is actually a DVD Read/Write station, as well as choices to startup from UBUNTU or Windows10.
Windows10 is on SDA with a UEFI Boot partition, on a 250GB HDD, #1 in the top of the list
Ubuntu is on SDB , on a 1 TB HDD , #2 in the list
The 3rd HDD has a 2TB capacity which I use as my Back-Up facility; it shows as #3 in the list
What I try to establish is automatically booting from #2 (UBUNTU) and not to enter the bios with every startup of my system
I see that your motherboard already support EZ Mode, then arranging your boot priority should be easy you only need to drag your ubuntu bootloader to the left. Assuming you already did that, and save (just to make sure), I see no reason why it would boot into windows again.
Here's a picture i found in your motherboard web page
pacanda :
By the way, after upgrading to UBUNTU 18.04 LTS shows GRUB2 no longer entry for Windows in the startup menu; only UBUNTU is visible.
Have you tried updating the grub from ubuntu? most of the time, it do the trick.
Simply load/open your windows partition (using explorer/caja or whatever they are called in ubuntu, since I used mint), then