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I have one of these with 5g and Wifi6 etc. There’s a warning that it doesn’t work in the US but I’m not clear whether that’s just 5g or whether 4g etc will work?

the device itself does not support a majority of the 5G or 4g LTE bands used in USA. none of the supported bands are "core" bands (core bands provide nationwide coverage, non-core bands used by a provider may not be available in your specific location)

best case you can use these bands below
5G/LTE band 41 (tmobile, if you find it, 30mbps-1000mbps. but not available everywhere and likely not have a good experience underground/inside buildings)
5G n77/78/79 (cband used by verizon/att/tmobile, quite rare, networks only started deploying it last year)
LTE band 26 or band 5 (tmobile kinda uses it, but sold it to dish and won't use it in future. AT&T/verizon uses band 5 a little)

AT&T is shutting down it's 3G network this month and Tmobile will shutdown 3G by July 2022