$\newcommand\Z{\mathbb Z}\newcommand\R{\mathbb R}$Let 
$$f(x):=\sum_{n\in\Z}K\Big(\frac{x-2^{-n}}{2^{-n-2}}\Big)
=\sum_{n\in\Z}K\Big(\frac{x-2^{-n}}{2^{-n-2}}\Big)1(x\in2^{-n}[3/4,5/4]),$$
where $K$ is a smooth nonzero function such that $K(x)=0$ if $|x|\ge1$. 

Then $f$ is smooth on $\R\setminus\{0\}$, $f(0)=0$, and $f(2x)=f(x)$ for all real $x$. So, for $n=2$,
$$\lim_{h\to 0}\frac{\sum_{k=0}^n f(x+k h)\binom nk (-1)^{n-k}}{h^n}$$ 
exists for all $x\in\R$, but $f$ is not even continuous at $0$.