Well you can run it from the installed system. That leads me to believe even more so that you have it installed in Bios mode but lets looks first.
This is the output of sudo fdisk -l :
Disk /dev/nvme1n1: 238,47 GiB, 256060514304 bytes, 500118192 sectors
Disk model: INTEL SSDPEKKW256G7
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 41E0AA89-BED6-4439-81D4-FBD2200E92A4
Device Start End Sectors Size Type
/dev/nvme1n1p1 2048 500117503 500115456 238,5G Microsoft basic data
Disk /dev/nvme0n1: 232,89 GiB, 250059350016 bytes, 488397168 sectors
Disk model: Samsung SSD 970 EVO Plus 250GB
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 2A0921FA-5590-43AD-B3A6-076AB5E2DADB
Device Start End Sectors Size Type
/dev/nvme0n1p1 2048 206847 204800 100M EFI System
/dev/nvme0n1p2 206848 239615 32768 16M Microsoft reserved
/dev/nvme0n1p3 239616 261875711 261636096 124,8G Microsoft basic data
/dev/nvme0n1p4 487155712 488394751 1239040 605M Windows recovery environment
/dev/nvme0n1p5 261875712 270264319 8388608 4G Linux swap
/dev/nvme0n1p6 270264320 487155711 216891392 103,4G Linux filesystem
Partition table entries are not in disk order.
It looks like it’s installed properly. 
So just a question then. You said you installed it just 4 days ago. Did it boot this way right from the beginning. Did you have any of the grub issues that others have had?
No I had no grub issue
Did it have all the entries in the grub menu? Windows, EndeavourOS, Fall back, and Firmware?
Yep, all the entries always present
So it will boot into windows from that grub menu? But not EndeavourOS and you can’t access the UEFI settings?
Could i see your hardware info? Just post the link.
inxi -Faz | eos-sendlog
The grub menu works properly except for UEFI settings. A small recap:
- I can’t access to uefi settings in any way
- Windows bootloader becomes the first in the boot order (so I don’t see the grub menu but it starts directly to windows) if: I push the “Restart” button instead of “Power off”; I try to access the UEFI firmware settings (both methods).
The link you asked:
Thanks. I looked at the hardware. UEFI Firmware is up to date. Everything looks fine. I think if it were me i would arch-chroot from the live ISO and reinstall grub and run the grub update command. That is the only thing i could recommend trying.
So you just boot on the live iso. Run the commands from the terminal. You have to mount the root of the installed EndeaourOS. Then mount the efi partition which is boot and then you arch-chroot to the mounted install.
Then you run the grub install command. Then run the grub update command.
sudo mount /dev/nvme0n1p6 /mnt
sudo mount /dev/nvme0n1p1 /mnt/boot/efi
sudo arch-chroot /mnt
You can run
ls /home
It should show give you your user name. Then you know you are properly chrooted.
Then run grub install ( Not sure you need sudo)
grub-install
Then run
sudo grub-mkconfig -o /boot/grub/grub.cfg
I’ll try this procedure asap. Thank you very much for your support 
The only result from this procedure is that the windows entry disappeared from the grub menu…
Don’t understand why that would happen. Are you able to access the firmware or still not?
Still not able
Well sorry that happened. It’s not what i was expecting. Does this laptop have a removable battery or not?
It’s removal but not easily, I have to unscrew the screws of the bottom case.
Anyway, I think I’ll just go back to Ubuntu which is more compatible with my laptop
Okay so i think if you can boot into EndeavouOS and remove all the UEFI entries. First see what is there using the command
efibootmgr
I read on the Acer site there is an issue with the exact problem you are having so clearing the UEFI entries should work. UEFI saves entries when it boots and or when you plug other bootable devices in. It sets the boot order also.
Edit: I’m not sure after if you’ll get the Windows entry back if this works…
Don’t worry, I can do a fresh install of windows if it’s necessary, I was prepared for that. Thank you again for your effort and for the time you lost, really appreciated 
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Well you do have that choice or you could try to run repair with the Windows disc after. Then Arch chroot again and do the process of installing grub and updating it to get the grub bootloader back. I think resolving the UEFI issue could happen with a reinstall maybe also. I would try removing some of the UEFI entries first and see if you can get it working first. It’s a good learning curve anyway as it helps you understand how your hardware works.
Edit: Again sorry for the troubles.
Don’t forget if you reinstall to make sure secure boot is disabled and CSM is also turned off. In Windows it is also advisable to make sure the fast startup feature in power management is also disabled as it uses hiber file and causes issues on dual boot.