This is a quick hack to build many-to-many relationships in Excel data model even though they are not supported out of the box.
Create intermediate calculated table and use DAX to fill it with unique values from related columns on both sides of the relationship:
EVALUATE FILTER( DISTINCT( UNION( VALUES('TableA'[Field]), VALUES('TableB'[Field]) ) ), NOT(ISBLANK([Field])) )
Create two one-to-many relationships placing this intermediate table in between
For all intents and purposes you may now forget that this intermediate table exists: it will get updated automatically whenever you update the data model, and it will not consume much resources. Pivot tables and DAX formulas will work as if the two original tables were directly connected via many-to-many link.
A similar intermediate table may be created with Power Query, but that's a lot less elegant (unless you're already using Power Query elsewhere in the workbook) and takes significantly longer to recalculate.