Separating C++ class into header(.h) and .cpp

Question

I'm currently trying to implement Priority queue using Binary search tree. I have the following header file:

template <class T>
class BSTQueue
{
private:
    struct Tree
    {
        T data;
        int priority;
        Tree* lt;
        Tree* rt;
        Tree(T Data, int Priority, Tree* left = nullptr, Tree* right = nullptr)
        {
            data = Data; priority = Priority; lt = left; rt = right;
        }
    };
    Tree* head = nullptr;
    Tree* insert(Tree* current, T data, int priority);
};

The problem is that function insert in .cpp generates errors. My .cpp file is the following:

#pragma once
#include "BSTQueue.h"

template<class T>
BSTQueue<T>::Tree* BSTQueue<T>::insert(Tree* current, T data, int priority)
{
    if (current == nullptr)
    {
        return new Tree(data, priority, nullptr, nullptr);
    }
    if (priority <= current->priority)
    {
        current->lt = insert(current->lt, data, priority);
    }
    else
    {
        current->rt = insert(current->rt, data, priority);
    }
    return current;
}

And the errors:

• Error C2447 '{': missing function header (old-style formal list?)
• Error C2143 syntax error: missing ';' before '{'
• Error C2061 syntax error: identifier 'Tree'

I have no idea what should I do in order to fix them. Maybe someone knows what to do??