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PSA: On Android: *#*#INFO#*#* then, Phone Information, Set Preferred Network Type: "LTE only". #defcon.
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It won't persist even within the same boot though. It only applies to the current connection to the current network. The infrastructure and user interface for the persistent state chooses to leave out support for LTE only even though it's fully supported and works as expected.
Persists fine for me, even through turning on/off airplane mode, and flying interstate across the US. I remained on the same network though (Google Fi).