Not saying I don't believe you, but it appears that your phone is connected to Wifi here. Are you sure this was from 5G and not your home Wifi?
Here is a screenshot of mine, its not the slowest I have seen, but this is fairly typical for me. I just tested it again and it came in at 5Mbps. Sometimes I do get the insane crazy fast 5G speeds, but more often its just standard 4G speeds. Notice how the icon on the phone claims to have full strength 5G service. To be honest I do suspect its more a fault of Google Fi and not necessarily T-Mobile, still a big disappointment either way.
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