There are four different places in the Book of Devarim that emphasize that the Mitzvot were meant to be observed in Eretz Yisrael.
In Chapters four, five, six, and twelve, the same theme is repeated. In each instance, Moshe is discussing the various commandments of the Torah. And each time, the wording is that these Mitzvot shall be observed in the land. On two occasions, the wording is, לעשות בארץ, “to do in the land.” Another time it says, לעשותכם אתם בארץ, that also means, to observe them in the land. And the fourth time, the wording is simply, ועשו בארץ, “they shall do it in the land.
What is clear from these four examples, is that G-d’s intention was for the Torah to be observed in Eretz Yisrael, and many Mitzvot could only be observed in the land.
This is the background to the famous Midrash based on the words from the second paragraph of Shema, ושמתם אותם, and you shall place these words (on your heart.)
The Sifrei writes that Hashem said to Israel: Even when you are in exile, you shall be excellent in the observance of Mitzvot, so that when you come back to the intended place of observance, they shall not appear as new to you.
This would indicate that the observance of Mitzvot outside of Israel, are just for practice. It also implies that it is a highly unnatural state for Jews to even be outside of Israel. In a normal, natural state, the Torah was the law of the land, and it was meant to be observed only in Eretz Yisrael.