Let $f : [0,2 \pi)^d \rightarrow \mathbb{R}$ be a square-integrable periodic function in $L^2( [0,2 \pi)^d )$ with $d \geq 1$.
We assume moreover that the square-summable Fourier coefficients of $f$, denoted by $c_n(f)$ for $n \in \mathbb{Z}^d$, are such that:

 1. $c_n(f)$ is real and strictly positive for any $n$,
 2. the sum $\sum_{n \in\mathbb{Z}^d} c_n(f) = \infty$, and
 3. the $c_n(f)$ are decreasing in the sense that $c_n(f) \leq c_m(f)$ as soon as $\lVert n \rVert \geq \lVert m \rVert$ for the Euclidian norm.

Can we prove that $f$ is necessarily discontinuous and even unbounded at $0$ with these assumptions?
  
*Context:* I am especially interested by the situation where $c_n(f)$ behaves asymptotically like $\lVert n \rVert^{- \alpha}$ for some $\alpha \in (1/2, 1]$.