  Enhancement of Curie temperature ($T_\mathrm{C}$) of two-dimensional (2D)
magnets is immensely desirable for room temperature spintronic applications.
Fe$_{5}$GeTe$_{2}$ is an exceptional van der Waals metallic ferromagnet due to
its tunable physical properties and relatively higher $T_\mathrm{C}$ than other
2D magnets. Using density functional theory combined with dynamical mean field
theory and Monte Carlo simulations, we show that the $T_\mathrm{C}$ of
Fe$_{5}$GeTe$_{2}$ monolayer can increase well-above room temperature by
substitutional doping with Ni. It is found that two specific sublattices (Fe1
and Fe4) are the first and second most energetically preferred occupation sites
for Ni. $T_\mathrm{C}$ of Fe$_{5-\delta}$Ni$_{\delta}$GeTe$_{2}$ increases up
to $\sim$ 400 K at $\delta \sim$20\%. Exchange interactions between particular
Fe5-Fe4 pairs play a dominating role in tuning the transition temperature,
influenced by doping-induced structural distortions. Finally, we highlight the
effect of dynamical electron correlation in site-specific electronic structure
and quasi-particle mass of Fe-$d$ orbitals with varying Ni dopi
