You have to have your template definitions available at the calling site. That means no .cpp files.
The reason is templates cannot be compiled. Think of functions as cookies, and the compiler is an oven.
Templates are only a cookie cutter, because they don’t know what type of cookie they are. It only tells the compiler how to make the function when given a type, but in itself, it can’t be used because there is no concrete type being operated on. You can’t cook a cookie cutter. Only when you have the tasty cookie dough ready (i.e., given the compiler the dough [type])) can you cut the cookie and cook it.
Likewise, only when you actually use the template with a certain type can the compiler generate the actual function, and compile it. It can’t do this, however, if the template definition is missing. You have to move it into the header file, so the caller of the function can make the cookie.