Why `NMaximize` function can't use nonnegative integer field as solution condition

Question

Maximize[{91 x1 + 71 x2 + 105 x3 + 103 x4 + 96 x5, 
  2.36 x1 + 2.12 x2 + 1.89 x3 + 3.77 x4 + 2.87 x5 <= 50, 0 <= x1 <= 3,
   1 <= x3 <= 5, 1 <= x4, 2 <= x3 + x4 <= 5}, {x1, x2, x3, x4, 
  x5}, NonNegativeIntegers]

Because the Maximize function sometimes returns the local optimal value, I want to use the NMaximize function to solve it. But the NMaximize function cannot use the non negative integer field as the solution condition:

NMaximize[-x^4 - 3 x^2 + x, x∈ 
      Reals]  
NMaximize[-x^4 - 3 x^2 + x, x∈ 
       NonNegativeReals]  
NMaximize[{91 x1 + 71 x2 + 105 x3 + 103 x4 + 96 x5, 
      2.36 x1 + 2.12 x2 + 1.89 x3 + 3.77 x4 + 2.87 x5 <= 50, 0 <= x1 <= 3,
       0 <= x2, 1 <= x3 <= 5, 1 <= x4, 
      2 <= x3 + x4 <= 5}, {x1, x2, x3, x4, x5} ∈ 
      NonNegativeIntegers]

Answer 1

Use {x1, x2, x3, x4, x5} ∈ NonNegativeIntegers as condition to NMaximize:

NMaximize[{91 x1 + 71 x2 + 105 x3 + 103 x4 + 96 x5, 
2.36 x1 + 2.12 x2 + 1.89 x3 + 3.77 x4 + 2.87 x5 <= 50, 0 <= x1 <= 3,
0 <= x2, 1 <= x3 <= 5, 1 <= x4, 
2 <= x3 + x4 <= 5, {x1, x2, x3, x4, x5} ∈ NonNegativeIntegers},
{x1, x2, x3, x4, x5}]

{1852., {x1 -> 3, x2 -> 0, x3 -> 4, x4 -> 1, x5 -> 11}}