Cross representations of additive complements of $r$-th powers

Let $\mathbb{N}$ be the set of natural numbers and $\mathcal{S}_r=\big\{1^r, 2^r, 3^r,\cdots\big\}$ the set of $r$-th powers, where $r\ge 2$ is a natural number. Let $\mathcal{W}_r$ be an additive complement of $\mathcal{S}_r$ and $$f_r(n)=\#\big\{(w,m^r)\in \mathcal{W}\times \mathcal{S}_r: n=w+m^r\big\}.$$ Motivated by a 1993 conjecture of Cilleruelo, we show that $$\sum_{n\le N}f_r(n)-N\gg_r N^{1-\frac{1}{r}}.$$ Previously, the bound was only proved for $r=2$. In the case $r=2$, the lower bound above can be made more explicit as $$\sum_{n\le N}f_2(n)-N\gg N^{1/2}(\log N)^{\delta}$$ for some absolute constant $\delta>0$, which improves a $\log$ factor upon a recent result of Ding, Sun, Wang and Xia.