My EVO 3D will randomly drop the 4g connection even when I am in great 4g coverage. THe actual 4g symbol goes away completely and I receive an error that I have no network connection. If I wait for a minute it comes back and I am good to go. This is not a huge problem but happens enough that I was wondering if it is a known issue or what the cause may be.
I would suggest pulling the battery and then doing a profile update. This can be done thru menu/settings/system updates/update profile. If the issue persists you may want to perform a factory reset on the device since it is happening in more than one location. A factory reset will restore the device to manufacturer default and everything will be wiped out besides what is stored on the SD card. You can also have your device diagnosed at a service center at no cost if you have TEP. Without it, you would pay 35.00.