Yankele, sorry, I misspelled RambaN 🙂
This starts with the discussion that Avos observed mitzvos but Yaakov married 2 sisters and made matzevah, Amram married his aunt …
Breishis 26:5
For the Commandments are the ordinance of the G-d of the land, even though we have been charged with personal duties in all places. Our Rabbis have already alluded to this secret, and I will yet call your attention to it with the help of G-d.
Vaikra 18:25
These [mitzvot] are not compulsory in the Exile with the exception of personal obligations, such as tefilin and mezuzot. And the Sages explain that this is so in order that [mitzvot] will not be novel to us upon our return to the Land of Israel, because the primary observance of all of the mitzvot [can be performed by] those living in G-d’s land.