Explain why $1055^{210} \not \equiv 1 \pmod {211}$.
The number 211 is prime.
We note that $1055 = 5 \times 211$. This means 211 is a factor of $1055^{210}$, and so
$1055^{210} \equiv 0 \pmod {211}$
This is why $1055^{210} \not \equiv 1 \pmod {211}$.