SImplest would be to separate the number and letter into two separate fields. Then you can sort first on the number field and then on the letter field.
If you keep this data in a single field, you'll need to sort on a calculationfield that appends some leading zeroes to keep the numerical part of the number to the same number of digits.
Let ( [ T = TextField ;
N = GetAsNumber ( T ) ;
L = If ( N ≠ T ; Right ( T ; 1 ) ; " " )
Right ( "000" & N ; 6 ) & L
The calculation assumes that no more than one character is used as a letter and will fail when the numerical portion exceedds 999,999.