I thought the clue might lie in the 26 column, but I listed all possibilities and can't eliminate any values:
Source: Puzzle Page app
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Compare the 15 column and the first three values of the 26 column.
One set must be the complement of the other, hence the sum of the first three values in the 26 column must be 15. This means the sum of the last two values is 11 and this can only be satisfied by choosing 9 and 2. I think it falls out from there.
Look at the eleven cells in the top right. From the column sums, we know they must add up to 78. But excluding the row of 42, the row sums only add up to 63, meaning that the two cells in that row add up to 15, and must be 7 and 8 respectively. This then lets you place the rest of the digits.