Rom 11:31 can clarify when the gathering of Israel that Paul speaks of in Rom 11:26.
Rom 11:31 says, “so they too have now been disobedient in order that by the mercy shown to you they also may now receive mercy.”
What’s interesting here is the last “now” in the verse. It seems to be saying that Israel will be gathered in at the same time as the Gentiles. This also goes along with the mechanism that Paul tells us will bring the Jews in – jealousy of the Gentiles (Rom 11:11) .
(the last “now” is found in Aleph, B, D, and other witnesses)
It also makes sense in light of Rom 11:12, “Now if their trespass means riches for the world, and if their failure means riches for the Gentiles, how much more will their full inclusion mean!”
Since Israel is seen here as blessing the Gentiles in their “full inclusion” we should not think that the full inclusion will happen after the Gentiles are gone as some do.