If there is a tower near you that can get a 4g signal, your phone is going to default to it because that's the default setting of the hardware. If you're in an area with low coverage, it may be that your roaming list needs to be updated using a "Hybrid PRL update" To do this power your device off, remove your sim card, re-insert sim card and power back on. The PRL update will update the list of towers your phone prefers to connect to in order to get the strongest signal possible.
wlb3890, it's important to us that you have reliable service. Your device is designed to retrieve signal from the best wireless tower in your area. As a result, it's very normal for service to fluctuate. Try powering your phone off/on to soft reset. Additionally, it's also helpful to enable voice roaming on in Settings>More Networks>Global Data Roaming>Allow Access for all trips. Keep us posted.
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I never understood the part about taking out and reinserting the sim with the power off. To me, reinserting after powering on made sense. Is there a bit of memory or power still used with power off?
It's because removing the sim with the device on can sometimes damage both the sim card and the device. So removing it while it's off is the only safe option
More than likely there is still some small amount of power still running through the device. That might also be why some phones are able to recognize multiple button presses(some require buttons other than the power button first) to boot a phone in recovery mode.