(On a side note: If this is somehow related to an earlier topic of yours, I'd recommend to answer my questions in that topic to help us helping you. Depending on those answers, there may be other ways to solve your issue without having to turn your RPi into a router. 
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Turning an SBC like an RPi into a router is completely unrelated to Pi-hole.
While it is definitive possible via more than one approach (e.g. by installing software like RaspAP or a full router OS), it would require you to decide on one such approach and then consult the respective documentation and support channels for that choice of yours.