Dell Inspiron 15 3552, changed boot mode to Legacy and back again, now it can't boot

Try the following.

https://neosmart.net/wiki/enable-uefi-boot/

https://www.dell.com/community/Inspiron/Used-Legacy-Boot-can-t-get-UEFI-w-Sec-Boot-back/td-p/5807045

Thank you for the advice, regretfully, both links haven't helped.

I've got settings back like it was suggested in the first link -- no results.

And the second link is about reinstalling, that I don't want to do. 

I'm attaching some images of my BIOS settings.

To be honest, I'm surprised to see "ubuntu" mentioned in the 4th image. The thing that started all this is that I've changed to Legacy to boot up from the ubuntu live usb stick, but I wasn't installing it, only running it live from the usb. In fact, that live stick even failed to bootup. But it looks that it was successful in ruining my Dell.

Any more suggestions? Please please please.

bios 2bios 3bios 4bootup error