Internet Speed is fast, but download speed on PC is really slow

The gateway's default network uses both 2.4 and 5 GHz (automatically selected by the gateway) and WPA2/WPA3 security.  This works really well for most things -- you won't need to try both 2.4 and 5 GHz on each of your connected devices to see which works better.  However, this might not be the best for older computers and devices.  You should add another network (use the + button at the bottom of the Network tab) and experiment with the settings for this second network to see if you can improve performance on your PC.  My desktop PC is really old, and only has 2.4 GHz.  So for it, I made a second network which is 2.4 GHz only.  I also found that it was much faster with the security set to WPA/WPA2.  I made this network Hidden -- because the security is downgraded, and no one else needs to see it.