It’s possible that grub is being installed incorrectly. I don’t have much experience with Gentoo, but I do with Linux and system recovery. If the machine is unable to find the bootloader then it’s likely not installed correctly.

You can try installing another distribution such as Mint or Ubuntu and see if it boots normally. If it does, then it’s indeed a grub installation/configuration problem. If not, then it’s a BIOS configuration issue.

Take note that the Gen7 also does not support UEFI, that wasn’t until the Gen9. So your partitioning format or set up may also(but rarely) cause problems.

If you haven’t figured it out already, make sure the the RAID volume you installed to is the controller’s selected boot volume. If you have multiple controllers, ensure the controller the drive is on is set as the first boot option.

If it’s that old, you’ll need an older browser. Internet Explorer would probably work fine. If you don’t have an old computer running something like Windows XP/Vista/early builds of Windows 7, you’ll need to create a virtual machine. Then sign into the dlink that way and look at the settings.