Using exchange rates

You need to make a couple of changes in your model. First, change the type of the [MRR] column to a decimal number. Also link 'Date'[Date] to 'Exchange Rates'[Time Period]. Then you can create a measure like

MRR Revenue Converted = 
VAR BaseTable = SUMMARIZE( 'Exchange Rates', 'Dates2'[Month Year], 'Exchange Rates'[Source], 'Exchange Rates'[Rate] )
VAR ConvertedValue = SUMX(
	BaseTable,
	VAR MaxDate = CALCULATE( MAX( 'Dates2'[Date] ) )
	VAR Source = 'Exchange Rates'[Source]
	VAR Result = CALCULATE(
		SUM( TblCombinedx[MRR] ),
		TblCombinedx[Calculated End Date] > MaxDate,
        TblCombinedx[(estimated) (billing) start date] <= MaxDate,
        TblCombinedx[PON/Other] IN { "PON", "Excluded" },
		TblCombinedx[Currency] = Source
	) * 'Exchange Rates'[Rate]
	RETURN Result
)
VAR UnconvertedValue = SUMX( VALUES( Dates2[Month Year] ),
	VAR MaxDate = CALCULATE( MAX( 'Dates2'[Date] ) )
	VAR Result = CALCULATE(
		SUM( TblCombinedx[MRR] ),
		TblCombinedx[Calculated End Date] > MaxDate,
        TblCombinedx[(estimated) (billing) start date] <= MaxDate,
        TblCombinedx[PON/Other] IN { "PON", "Excluded" },
		ISBLANK( TblCombinedx[Currency] )
	)
	RETURN Result
)
VAR Result = ConvertedValue + UnconvertedValue
RETURN Result

The line

TblCombinedx[Currency] = Source

makes sure that it is the correct currency.