Modulo arithemtic means that you should use corresponding remainder, not the value itself.
In your case:
5 + 25 (mod 26) == 30 % 26 == 4 // <- "%" is taking remainder after dividing on 26
Whatever operation you do in mod 26, the answer will be in 0..25 range.
Sample code (C#):
int x = 5;
int y = 25;
int mod = 26;
int result = (x + y) % mod;
solved How to do modulo calculation? [closed]