I am having this problem too in Seattle, and I see it hasn't been fixed in 1½ years. I have to either reboot or wait several minutes for 4G coverage to come back. This happened with at least 3 different SIM cards. Please make sure troubleshooting this problem is still on the table.
Let’s get a better connection for your phone MindDrive. We’ll start off with a few questions. What zipcode are you having the trouble in? What make/model phones are affected?
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Zip code 98134. Definitely good coverage here. But when I use the elevator to leave work, it understandably drops to 3G coverage, then takes several minutes or a restart (or evidently cycling in and out of airplane mode) before restoring 4G coverage.