The location may have been in Phoenicia (coast of modern Syria) for two reasons.
First, the word in the Great Isaiah Scroll of the Dead Sea Scrolls appears different. That is, instead of סינים (as received in Masoretic Text) the word is סוניים.
In other words, the word סוניים would appear to be the plural form of the word סיני, which is found in Gen 10:17 and 1 Chr 10:17, which refer to the Canaanite patriarch. That is, if the yod is taken for the waw (according to Falk et al.), then the word סוניים may be read as סיניים , which is the plural of סיני.
In this regard, the word סיניים would mean “Sinites,” who were Canaanites who lived north and west of the Land of Israel. According to Fisher et al., archaeological discoveries of Ugaritic tablets with transcriptions aligned with this Hebrew word indicate that locations associated with this name were in what is now northwest Syria.
Secondly, the logical dichotomy of the editing of cantillation in the Masoretic Text indicates that the phrase “And lo, these will come from the north and from the west” (illustrated by the blue box, below) is modified by the phrase “And these from the land of Sinim” (illustrated by the red box, below).
In other words, the “land of Sinim” is Canaanite land “north and west” of the Land of Israel, which is the coast of modern Syria today.