Query optimizer recommends adding index instead of using existing index
Your index is seemingly fine and good (i.e. covering) for the query and it should be used. The real problem is the query itself and specifically this condition which hides an implicit conversion:

WHERE [serialNumber] = 137802


According to SQL Server's datatype precedence, when two values of different datatypes are compared, the value with the datatype of lower precedence is converted to the datatype of the higher precedence. Unfortunately, int is higher in the list than varchar. This blows up any hope of using the index as the column (serialNumber) values are converted to integers. The column being the 1st position of the index, leads the optimizer to not use that index and search for an alternative (and thus the suggestion.)

Solution is not to have any implicit or explicit conversions of columns in WHERE condition. Simply use:

WHERE [serialNumber] = '137802'