Take b = 2.

b^(n-1) mod n = 1.

21143 is prime.

b^((n-1)/21143)-1 mod n = 196245148, which is a unit, inverse 504004310.

1373 is prime.

b^((n-1)/1373)-1 mod n = 880858, which is a unit, inverse 344921177.

(1373 * 21143) divides n-1.

(1373 * 21143)^2 > n.

n is prime by Pocklington's theorem.