It's simple really, all the commandments and instructions that relate to the Temple, are impossible to perform, therefore you are actually KEEPING the mitzvot by NOT performing Temple services OUTSIDE the Temple.
Read Deuteronomy 30:1-5~ God expects His people to obey His commandments, even when they are SCATTERED among heathens. (no temple, no priesthood, no land of Israel, no theocratic judicial system)
""So it shall be when all of these things have come upon you, the blessing and the curse which I have set before you, and you call them to mind in all nations where the LORD your God has banished you, and you return to the LORD your God and obey Him with all your heart and soul according to all that I command you today, you and your sons, then the LORD your God will restore you from captivity, and have compassion on you, and will gather you again from all the peoples where the LORD your God has scattered you." - Deut. 30:1-3
Doesn't matter where you are.
Doesn't matter if there is or isn't a Temple.
Doesn't matter if there is or isn't a Levitical Priesthood.
Doesn't matter if there is or isn't a Theocratic Judicial system in the Land.
As a child of God, God wants you to obey the commandments of God!
you know why?
because~ "this commandment which I command you today is not too difficult for you, nor is it out of reach. It is not in heaven, that you should say, `Who will go up to heaven for us to get it for us and make us hear it, that we may observe it?' Nor is it beyond the sea, that you should say, `Who will cross the sea for us to get it for us and make us hear it, that we may observe it?' "But the word is very near you, in your mouth and in your heart, that you may observe it." - Deut. 30:11-14