[Q] 2G - 4G network switching

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http://forum.xda-developers.com – does anybody know how to switch between 2g and 4g network in galaxy s3? earlier when i was using stock rom it used to automatically switch between 2g and 4g depending on network strength.. after that i installed cm 10.1 and i selected 'use only 2g network' option, and for some reason i reverted back to stock rom.. since then its staying in 2g mode, i tried so many things but not able to switch to 4g or 3g... so is there any way to change the networks?? (HowTos)