Originally Posted by SPARTAN_TECH
So I bought a new HTC one sprint variant have root s off and sim unlock successful what am I missing to get straight talk att sim to work
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TBH, you'd be better off buying a HTC One ATT/T-Mo variant since (I think, but haven't looked up) all frequencies work on it.
Anyways, you should be able to just insert the SIM and you should be good to go. Make sure you go into your Mobile Network settings, and change the mode to GSM. Alternatively you could go into testing (*#*#4636#*#*) and change the mode to something like WCDMA preferred.