Inspiron 3670, Ubuntu, no bootable device found

Hi everyone,

I tried installing Ubuntu along with windows 10 in my Inspiron 3670 in legacy mode (since the option to boot from USB was not coming in the UEFI mode). However, after installation, the Ubuntu would not load. So, I installed Ubuntu next time while erasing the disk hoping that it would at least boot. But to my surprise, it did not boot and the message of "no bootable device found" flashed. I have tried repairing (and installing the grub) using Ubuntu boot-repair but even that did not help. The windows boot manager is also not being displayed in the UEFI boot option and if I try to add a new boot option, it says "file not found". Kindly help me fix this problem. The entire HDD and SSD have been formatted and all the partitions have been removed. Any leads would be appreciated.