fedora’s grub/os-prober isn’t compatible with Arch. Arch uses multiple initrd images which Fedora’s grub/os-prober doesn’t support. Arch’s grub/os-prober supports both so you should either use that grub or add a manual entry for Arch. Make sure you add an initrd line for you ucode if you go that route.
As for secure boot, if it is a standard Arch install you will need to disable secure boot. I don’t think using Fedora’s grub will allow you to boot an unsigned kernel.