Tachanun and local custom

Hi,

One who lives in a place where Tachanun is recited without the 13 attributes of Divine mercy, and who prays at some point with a minyan which does recite them, is obligated to recite them with the community. One may not diverge visibly from communal custom.

(Rav Moshe Feinstein, Igrot Moshe Orach Chaim 3:89)

Have a great day,
Mordechai