Create custom index

Below a quite advanced Power Query (M) Solution using List.Accumulate.

Note 1: if your table is very large, then I would recommend to split it in a table with Orders that have no Previous or Next Order number and a table that does have related orders.

Note 2: I included column RowID in the table and used it at the end of the query to sort the result back to the original sort order.

Add a column with nested lists, each containing the previous and next order number.

Add a column with grouped orders (which is the heart of the solution):

the first argument of List.Accumulate is the complete column that was previously added

the second argument is the start value, being a list with current order no and its previous and next order numbers (nulls removed)

Now List.Accumulate loops over the list with previous and next order numbers, if any of these numbers are already in the list (GroupSoFar), than both are added to the list (sorted, only distinct values and nulls removed).

Now you can extract the order numbers to a comma separated field, group on this column (with operation All Rows to get all data in nested tables  in column "AllData") and proceed with the rest of the code (which is quite straightforward).

let
    Source = ServiceOrders,
    AddedOrdersList = Table.AddColumn(Source, "OrdersList", each {[#"Prev. Order No."], [Next Order No]}),
    AddedOrdersGrouped = Table.AddColumn(AddedOrdersList, 
                         "OrdersGrouped", 
                         each List.Accumulate(AddedOrdersList[OrdersList],
                                              List.RemoveNulls({[Order No], [#"Prev. Order No."], [Next Order No]}), 
                                              (GroupSoFar,NewMembers) => 
                                                List.Sort(List.Distinct(GroupSoFar & 
                                                                        (if List.ContainsAny(GroupSoFar,NewMembers) 
                                                                            then List.RemoveNulls(NewMembers) 
                                                                            else {})
                                                                        )))),
    #"Extracted Values" = Table.TransformColumns(AddedOrdersGrouped, {"OrdersGrouped", each Text.Combine(List.Transform(_, Text.From), ","), type text}),
    #"Removed Columns" = Table.RemoveColumns(#"Extracted Values",{"OrdersList"}),
    #"Grouped Rows" = Table.Group(#"Removed Columns", {"OrdersGrouped"}, {{"AllData", each _, type table}}),
    #"Added Index" = Table.AddIndexColumn(#"Grouped Rows", "Index", 1, 1),
    #"Expanded AllData" = Table.ExpandTableColumn(#"Added Index", "AllData", {"RowID", "Order No", "Prev. Order No.", "Next Order No", "Service Time (hours)"}, {"RowID", "Order No", "Prev. Order No.", "Next Order No", "Service Time (hours)"}),
    #"Removed Columns1" = Table.RemoveColumns(#"Expanded AllData",{"OrdersGrouped"}),
    #"Reordered Columns" = Table.ReorderColumns(#"Removed Columns1",{"RowID", "Order No", "Prev. Order No.", "Next Order No", "Index", "Service Time (hours)"}),
    #"Sorted Rows" = Table.Sort(#"Reordered Columns",{{"RowID", Order.Ascending}})
in
    #"Sorted Rows"

Eric_Zhang, thank you! that works perfectly.

MarcelBeug ,thank you as well, I will try your solution as soon as I can and will send my feedback!

Again, thank you guys for your help

Barna