It looks like the black box in your photo is a combination fiber interface and router. If you haven't placed this device into "bridge" mode, then all the devices on your LAN will be double NAT'd.
Double NAT should not cause any problems with devices reaching the Internet, but will cause problems if you want to connect to your home network remotely. For example if you have a file or video server, or perhaps a VPN server running on your router.
Assuming these double NAT issues do not apply to you, then we can ignore the double NAT for now.
Since all the other devices on your network can reach the Internet, and this PC is visible on the LAN, (it would be interesting how you determined that), the first thing that comes to mind is that the PC does not have the correct gateway address.
Go into the PC's network properties, and make sure it is set to "obtain an address automatically".
Try to ping 8.8.8.8. If you get a successful reply, then your DNS settings on the PC may be incorrect.