Non-isomorphic metacyclic $p$-groups of split type with the same group zeta function

For a finite group $G$, let $a_n(G)$ be the number of subgroups of order $n$ and define ${\zeta }_G(s)={\sum }_{n\ge 1}{a}_n(G)n^{-s}$. Examples are known of non-isomorphic finite groups with the same group zeta function. However, no general criterion is known for when two finite groups have the same group zeta function. Fix integers $m,n\ge 1$ and a prime $p$, and consider the metacyclic $p$-groups of split type $G(p,m,n,k)$ defined by $G(p,m,n,k)=⟨a,b\mid a^{p^m}=b^{p^n}=\mathrm{i}\mathrm{d},b^{-1}ab=a^k⟩$. For fixed $m$ and $n$, we characterize the pairs of parameters $k_1,k_2$ for which ${\zeta }_{G(p,m,n,k_1)}(s)={\zeta }_{G(p,m,n,k_2)}(s)$.