Bicycle question road bikes?

my road bike i bought had a bummed rear deraiuler/fgear changer well it was more like i couldnt figure out how it went back on as its completly differnt from a modern setup i found a newer mtb bike that was a piece of junk but it happend to be the same gear setup front and rear it also had 2 front... show more my road bike i bought had a bummed rear deraiuler/fgear changer well it was more like i couldnt figure out how it went back on as its completly differnt from a modern setup i found a newer mtb bike that was a piece of junk but it happend to be the same gear setup front and rear it also had 2 front gears and 5 rear gears i swapped in the gearchanger for the rear and before my tire blew i was only getting 3 out of 5 gears i may have mis adjusted the derailer when i was trying to get the other gears to work or it may have been mis adjusted from the start as i got it off a junk bike my question is do you think i dont have all the gears because its misadjusted or because its a newer part?
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