We present the analysis of the fundamental plane (FP) for a sample of 19 massive red-sequence galaxies ($M_{star} >4times10^{10} M_{odot}$) in 3 known overdensities at $1.39<z<1.61$ from the KMOS Cluster Survey, a guaranteed time program with spectroscopy from the K-band Multi-Object Spectrograph (KMOS) at the VLT and imaging from the Hubble Space Telescope. As expected, we find that the FP zero-point in $B$ band evolves with redshift, from the value 0.443 of Coma to $-0.10pm0.09$, $-0.19pm0.05$, $-0.29pm0.12$ for our clusters at $z=1.39$, $z=1.46$, and $z=1.61$, respectively. For the most massive galaxies ($log M_{star}/M_{odot}>11$) in our sample, we translate the FP zero-point evolution into a mass-to-light-ratio $M/L$ evolution finding $Delta log M/L_{B}=(-0.46pm0.10)z$, $Delta log M/L_{B}=(-0.52pm0.07)z$, to $Delta log M/L_{B}=(-0.55pm0.10)z$, respectively. We assess the potential contribution of the galaxies structural and stellar velocity dispersion evolution to the evolution of the FP zero-point and find it to be $sim$6-35 % of the FP zero-point evolution. The rate of $M/L$ evolution is consistent with galaxies evolving passively. By using single stellar population models, we find an average age of $2.33^{+0.86}_{-0.51}$ Gyr for the $log M_{star}/M_{odot}>11$ galaxies in our massive and virialized cluster at $z=1.39$, $1.59^{+1.40}_{-0.62}$ Gyr in a massive but not virialized cluster at $z=1.46$, and $1.20^{+1.03}_{-0.47}$ Gyr in a protocluster at $z=1.61$. After accounting for the difference in the age of the Universe between redshifts, the ages of the galaxies in the three overdensities are consistent within the errors, with possibly a weak suggestion that galaxies in the most evolved structure are older.
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