Cheats, yeah. Regular 21st century knowledge? I'm not so sure. I think only a few specific fields of expertise would actually be usable in a mediaeval context. You could be the world's greatest computer scientist or nuclear physicist, but those skills won't be worth shit in a mediaeval fantasy isekai. You might even get labelled a lunatic or a demon if you ever talk about your knowledge in those fields. The most effective way to get rich from nothing was probably to become a merchant, but that takes a lot of interpersonal skills, so just having e.g. a degree in economics probably wouldn't be much use. It's another world, not Earth, so knowledge of history would be largely useless, etc. etc. Moreover, most people today are lacking very fundamental skills that would be required to live in a mediaeval era, like how to farm (over 90% of the population were farmers)—till land, rotate crops, harvest and store your corn, etc.—, how to make clothes, how to hunt and butcher small game, how to build a shelter, and so on.
quite the contrary, basic high-school knowledge would be revolutionary in quite a few fields depending on the period
-germ theory, and the knowledge of lenses to prove it
-basic medical and anatomical knowledge, bonus points if you remember how to isolate penicillin or synthesize sulfa drugs
-boiling and other methods of purifying water at scale (and why it works)
-pasteurization alone could set you up for life
-math up to calculus
-movable type
-paper making
-steam power and the use of coal, oil, and natural gas
-basic metallurgy
-physics up to newton's laws
-astronomy
-a general idea of how to make guns, particularly self-contained cartridges
-saddles, stirrups an other kit for horses
-crop rotation, fertilizers (potash being a good example)
-sewage systems and garbage disposal particularly their link to common diseases
-concrete making and use
-the list goes on
even if you were say a nuclear physicist you'd easily blast away the scholars of the day and your basic foundational coursework would gradually put the nation you supported leagues ahead of its peers, which would only accelerate once you started teaching others.
