Finding paths connected through a node. Data filtering

Try

Connected Mains = 
VAR InitialNodes =
	UNION( VALUES( 'Table'[Str1] ), VALUES( 'Table'[Str2] ) )
VAR ConnectedMains =
	CALCULATETABLE(
		VALUES( 'Table'[Main] ),
		'Table'[Str1] IN InitialNodes
			|| 'Table'[Str2] IN InitialNodes,
		REMOVEFILTERS( 'Table' )
	)
VAR ConnectedMainsExlcudingCurrent =
	EXCEPT( ConnectedMains, { SELECTEDVALUE( 'Table'[Main] ) } )
VAR Result =
	CONCATENATEX(
		ConnectedMainsExlcudingCurrent,
		'Table'[Main],
		", "
	)
RETURN
	Result

If I understand correctly you want a list of mains and all the nodes they are connected to. You could do another table like

Main All Connected =
GENERATE (
    ALLNOBLANKROW ( 'Main Neighbours'[Radialnr-rettet] ),
    DISTINCT (
        UNION (
            CALCULATETABLE ( VALUES ( 'Main Neighbours'[Connected To] ) ),
            CALCULATETABLE ( VALUES ( 'Main Neighbours'[Neighbours Neighbour] ) )
        )
    )
)