Hello,

Yes. You will be eligible.

For IISER , you must pass Class 12 in the year of admission or one year before it.
If you want to appear for IISER 2027, your Class 12 pass year must be 2026 or 2027 .

Since you passed Class 12 in 2024 , that pass is not valid for IISER 2027.
If you pass Class 12 again through NIOS in 2026 , your latest pass year (2026) will be considered.

So, by passing 12th again with NIOS in 2026, you will be eligible for IISER 2027.

Hope it helps!