Google the kind of phone you have and ATT is your provider I take it? Include which version of android also so something like..... i337 modem att android 4.3 of course swtich the proper name and provider that you have. You don't want to be flashing the wrong modem you could end up with a paper weight....
Telus Galaxy S4 - Stock 4.3 to
[ROM] KOT49H【4.4.2】Google Edition【AOSP Features】【Optimized】02-06-2014