I don’t think you really can. The thing is both a dremel and a router have a high RPM rate. But their amperage is dramatically different: a dremel has a maximum of 1,5 amperes, while a router can offer as much as 7 amperes.
So, theoretically, you can use a dremel for some routing. But in reality, it will be a slow, arduous, and painful process. It will also lead to quicker degradation of your device, as it’s simply not powerful enough.
By the way, Dremel is a nickname for every rotary tool that has a high RPM rate. But it’s also an actual brand that offers a whole kit of attachment pieces that can turn a rotary gadget into an improvised router. But only time will show if it’s efficient enough.