I have try to download iOS 15.4 over 10hours , it only say updated requested
iPhone XS Max, iOS 14
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You must have a stable Wi-Fi connection.
Or connect to a PC with the latest version of iTunes installed.
Charge the battery to 50% or higher.
Turn off Battery saving mode.
Don't use a Hotspot.
Don't use VPN.
And make sure you have enough free space.