looking up values where two ranges apply

Another option is to start with the Orders table, merge in the Customer table (matching on the Customer column), expand the date and unique reference columns, and then filter out the rows that don't satisfy the time overlap.

Here's some example code:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WclTSUbK0MDcFUib6hvpGBkZGQKYZjBmrA1ViaWgOpMz1TWBKTPVNoUpiAQ==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Customer = _t, Order = _t, #"Start Date" = _t, #"Order End" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Customer", type text}, {"Order", Int64.Type}, {"Start Date", type date}, {"Order End", type date}}, "en-IN"),
    #"Merged Queries" = Table.NestedJoin(#"Changed Type", {"Customer"}, Customers, {"Customer"}, "Customers", JoinKind.LeftOuter),
    #"Expanded Customers" = Table.ExpandTableColumn(#"Merged Queries", "Customers", {"Start Date", "End Date", "Unique Reference"}, {"Customers.Start Date", "Customers.End Date", "Customers.Unique Reference"}),
    #"Filtered Rows" = Table.SelectRows(#"Expanded Customers", each ([Customers.Start Date] <= [Order End] and [Customers.End Date] >= [Start Date]))
in
    #"Filtered Rows"