Pesachim 50b: If one goes from a place where they work on Erev Pesach morning to a place where they do not work, or vice versa, he must follow the stringencies of both places.

Shulchan Aruch Orach Chaim 496:3: Residents of Eretz Yisroel who came to Chutz Laaretz are forbidden to do work on Yom Tov Sheini when in a city, even if they intend to return.

פסחים נ ע”ב: ההולך ממקום שעושין למקום שאין עושין או ממקום שאין עושין למקום שעושין נותנין עליו חומרי מקום שיצא משם וחומרי מקום שהלך לשם.

שו”ע או”ח תצ”ו ס”ג: בני ארץ ישראל שבאו לחוצה לארץ אסורים לעשות מלאכה ביום טוב שני בישוב אפילו דעתו לחזור.

A man from Eretz Yisroel who was staying in Antwerp for Yom Tov came over to Rav Chaim Kreiswrith and asked, “I just put on tefillin on Yom Tov Sheini. Was that correct?” Rav Kreiswirth refused to answer, saying, “I don’t answer questions about something that was already done. Siyata dishmaya is given to a posek only on questions that are relevant to someone’s future actions.”

Source: Mayim Chaim, p. 173

[According to the Shulchan Aruch, who holds that a Ben Eretz Yisroel keeps the Yom Tov Sheini in Chutz Laaretz only as a chumra, clearly he would be obligated to be machmir the opposite way as well, and put on tefillin.

However, the Chacham Tzvi (167) disagrees. He writes that minhag hamakom applies only in questions of halacha, where the same halacha applies in all locations, yet different communities follow different opinions. For example, the Gemara says that Eretz Yisroel and Bavel followed two different practices as to whether a certain fat is permitted. What is forbidden fat is forbidden everywhere, but certain places had a minhag to follow the opinion that considers certain fats not forbidden. If the community that considered it forbidden were to move en masse to the other location, they would continue to refrain from eating it, because the prohibition is not dependent on location. But Yom Tov Sheini is not a universal halachic question; it simply depends on whether the messengers of Beis Din reached that particular place. Therefore, people from Chutz Laaretz who are staying temporarily in Eretz Yisroel should only keep one day Yom Tov, just as they would if they had visited during the time when Beis Din sent out messengers.

According to this Chacham Tzvi, a resident of Eretz Yisroel while visiting Chutz Laaretz he must act completely like the local people, which would mean he does not put on tefillin either. Perhaps this was the question the man wanted to ask Rav Kreiswirth.]