已知\(λ < 0 \)，数列\(\left\{{a}_{n}\right\} \)满足\({a}_{n+1}-λ{a}_{n}=λ-{λ}^{2}\left(n∈{N}^{*}\right),且{a}_{1}=3λ \)．

\((1)\)证明：数列\(\left\{{a}_{n}-λ\right\} \)是等比数列；

\((2)\)若对任意\(m,n∈{N}^{*} \)都有\(-λ < \dfrac{{a}_{m}}{{a}_{n}} < - \dfrac{1}{λ} \)，求实数\(λ \)的取值范围