Hi,
Not sure which GL503V model you have, but according to the global website specs below, looks like it should be supporting NVMe drives.
https://www.asus.com/ROG-Republic-Of-Gamers/ROG-Strix-GL503/Tech-Specs/
You will not normally see NVME drives listed in the bios with the SATA devices though, so if you can't find another section for IRST or NVMe drives, it may be possible your bios just doesn't list them separately.
If you have a Windows installation drive around and haven't tried booting to it yet, you may want to check if you can still partition and install Windows to your NVME drive despite what the bios shows?
If Linux was installed on the NVME drive, you may need to find a way to format it so that the Windows installation disk can recognize it though.
Otherwise, if you have a repair shop that you trust, you may want to consider asking them to look into it more.