Need help with DAX code for cumulative values

honeybee84 , Try using

Cumulative Prior Month Forecast =
VAR SelectedReportingPeriod = MAX(FACT_CombinedData[Reporting Period])
VAR PriorReportingPeriod = EDATE(SelectedReportingPeriod, -1) // Get the prior reporting period

RETURN
CALCULATE(
SUMX(
FILTER(
ALL(DIM_Date),
DIM_Date[FullDateAlternateKey] <= SelectedReportingPeriod &&
DIM_Date[FullDateAlternateKey] >= PriorReportingPeriod // Ensures cumulative roll-up from prior reporting period
),
[Forecast] // Accumulates forecast values month by month
),
TREATAS(
{PriorReportingPeriod}, FACT_CombinedData[Reporting Period] // Passes filter without needing an active relationship
)
)

You can try

Cumulative Prior Month Forecast =
VAR SelectedMonthDate =
    MAX ( DIM_Date[FullDateAlternateKey] )
VAR DatesToUse =
    CALCULATETABLE (
        DATESYTD ( DIM_Date[Year Month] ),
        TREATAS (
            { EOMONTH ( SelectedMonthDate, -2 ) + 1 },
            DIM_Date[FullDateAlternateKey]
        )
    )
VAR Result =
    SUMX ( DatesToUse, [Forecast] )
RETURN
    Result

where DIM_Date[Year month] is the date column linked to your fact reporting period column.