A hidden network does not show up in the normal list. You must enter the SSID manually along with the security type and password.
What you need
- Exact network name (SSID), including capitalization
- Password
- Security type (usually WPA2 or WPA3)
iPhone
- Settings → Wi-Fi → Other…
- Enter name, security, and password → Join.
Android
- Settings → Network → Internet → Add network (or similar).
- Enter SSID, security, password → Save.
Windows
- Settings → Network & internet → Wi-Fi → Manage known networks → Add network.
- Enter name, security type, password. Check Connect even if this network is not broadcasting if shown.
Mac
- Wi-Fi menu → Other Network…
- Enter name, security, password → Join.
Related: how to hide SSID, find Wi-Fi password.