Joining two tables where TABLE1.Datetime is between TABLE2.StartDatetime and TABLE2.EndDatetime

See if this helps. It returns this in Power Query:

To get this result, add a column to your Table 1 with this formula:

Table.SelectRows(
    Table2,
    (Ranges) => ([Datetime] >= Ranges[StartDateTime])
                and
                ([Datetime] <= Ranges[EndDateTime])

)

That will return a nested table from Table2 that has only the records in that range.

Then just expand that added custom column.

See this file for full details.