The 2.4 GHz network appears in the Wi-Fi list, but the 5 GHz network does not. The two bands are broadcast separately, so a missing 5 GHz network usually means the router has the band off, the device cannot use 5 GHz, or the band is using a channel the device cannot see.
The fix, in order
Check whether the adapter supports 5 GHz. On Windows, open Command Prompt and run netsh wlan show drivers. Look for the supported radio types: 802.11a, 802.11ac, or 802.11ax mean 5 GHz is supported, while 802.11b/g/n only means 2.4 GHz only.
Check the router settings. Log into the router and make sure the 5 GHz band is enabled and broadcasting its name. Some routers combine both bands under one name, in which case there is no separate 5 GHz network to find.
Separate the band names. If the router uses one name for both bands, give the 5 GHz network its own name so you can see and choose it directly.
Change the 5 GHz channel. Some devices cannot see higher 5 GHz channels. Setting the router to channel 36, 40, 44, or 48 is the most compatible choice. Use the Wi-Fi channel planner to pick a channel with the least interference.
Update the Wi-Fi driver. An old driver may not list 5 GHz networks even when the hardware supports them. Install the latest driver from the computer maker's site.
Move closer to the router. 5 GHz has a shorter range than 2.4 GHz and is blocked more easily by walls, so it may simply be out of reach where 2.4 GHz still works.
Do not buy a new router just for 5 GHz until you have checked the adapter. Many older laptops and budget devices only have 2.4 GHz radios, and no router setting will change that.
If it still happens
If the adapter supports 5 GHz and the router broadcasts it, but the network still does not appear, try a different 5 GHz channel and check for a router firmware update. A USB Wi-Fi adapter that supports 5 GHz is a cheap way to confirm whether the built-in adapter is the problem.
Common questions
What is the difference between 2.4 GHz and 5 GHz Wi-Fi? 2.4 GHz reaches further and passes through walls better, but it is slower and more crowded. 5 GHz is faster and less crowded, but its range is shorter and it is blocked more easily.
Why does 5 GHz disappear in some rooms? 5 GHz signals weaken faster through walls and floors than 2.4 GHz signals. A network that is strong in the same room as the router can vanish two rooms away.