There are at least two ways to install OpenWrt on an x86 device with a SATA drive.
Method 1 (requires a USB-to-SATA cable). Take the SATA drive out of your device, download OpenWrt to your computer (your actual computer, not the machine you're converting into a router), connect the SATA drive to your computer's USB port (that's where the cable comes in), and write OpenWrt directly from the image onto the SATA drive using any image writing program (if you're on Windows, Rufus does this very well; if you're on Linux, you can use the good old dd on the command line; Etcher exists on both platforms; there are other options as well). Then, put the SSD back into your device, and it should start right away. Here's an example of how this is done (yes, I wrote this, so consider it shameless self-promotion):
If you decide to follow this example, change generic to 64 throughout the example.
Method 2 (requires a USB stick). Download OpenWrt to your computer and write it to the USB stick using one of the programs mentioned above (Rufus, dd, Etcher). Next, boot your device from the USB stick. When OpenWrt starts, download another copy of OpenWrt, unzip it, and write it to the SSD. Here's an example (yes, I wrote this one, too, so more shameless self-promotion):
Note that your device is different from the ones discussed in the examples, so be sure to do the investigative steps before applying the fixes. Also note that both of these examples use console; in your case, it's probably easier to connect a monitor and a keyboard to your router-to-be.
If you get stuck, post a note here on the forum. Give as much detail as you can about what you were doing, what you expected to happen, and what actually happened.