Phone stuck on 3G

Here are a few reasons why a phone can be stuck on 3G (3G/H/H+):

That 3G is connected. In order for 3G to be upgraded to 4G, it must first be disconnected, such as due to inactivity. This means that when 3G is kept active, such as during the streaming of music, 3G cannot be upgraded.
That 4G/5G is not enabled in the settings, or are effectively disabled in any band/RAT locking utility.
That the 3G tower did not describe 4G. How a phone switches is based on settings loaded from the tower. If *#0011# displays SIB19:0 or SIB19 Not Received, the tower did not describe 4G or the phone skipped it e.g. because 4G is disabled in the settings.
That the minimum criteria of 4G are not met. For example, even if 4G is usable, it being below a minimum signal strength threshold will cause it not to be reselected to.
That the carrier sent the equivalent of 'you were logged off due to...' / 'logging in failed due to...' and the reason prevents use of 4G/5G.
That the 4G towers forbid entry.
That the 4G signal strength, signal quality, or range is inadequate.
That the SIM is not a R99 USIM or newer.