This can be done via mission select if you happen to not get this when playing.
During the mission Identity Crisis you will be asked a series of questions by Adler, the answers don't matter too much until Adler asks you "Where is Persus?" You will then have two options to pick from that are highlighted in yellow text. You will want to pick the option "[Truth] Solovetsky. It's Solovetsky Monastery"
This will unlock the The Final Countdown mission, do the same if you need to complete Ashes to Ashes by selecting "[Lie] Duga, The radar array near Duga."
Hope this helped.