For the modem, find a copy of MK6 modem assuming your in Canada, flash it through odin from your pc. If you get a fail, chances are you have a bad download (happened to me) download from a different source then. Before you flash any rom... root.... install custom recovery.... nand backup your current "stock but rooted rom" If you flash a 4.4 rom (not sure if this applies to 4.3) do a full wipe... System/cache/dalvic.... then factory reset... then flash.... I flashed 4.4 the first 2 times without wiping system and though the phone worked fine, data was inconsistant at times.....
Telus Galaxy S4 - Stock 4.3 to
[ROM] KOT49H【4.4.2】Google Edition【AOSP Features】【Optimized】02-06-2014