  We study the possibility of right-handed neutrino dark matter (DM) in gauged
$U(1)_{B-L} \times Z_2$ extension of the standard model augmented by an
additional scalar doublet, being odd under the $Z_2$ symmetry, to give rise the
scotogenic scenario of radiative neutrino masses. Due to lepton portal
interactions, the right-handed neutrino DM can have additional co-annihilation
channels apart from the usual annihilations through $Z_{B-L}$ which give rise
to much more allowed mass of DM from relic abundance criteria, even away from
the resonance region like $M_{\rm DM} \approx M_{Z_{B-L}}/2$. This enlarged
parameter space is found to be consistent with neutrino mass constraints while
being sensitive to direct detection experiments of DM as well as rare decay
experiments looking for charged lepton flavour violating decays like $\mu
\rightarrow e \gamma$. Due to the possibility of the $Z_2$ odd scalar doublet
being the next to lightest stable particle that can be sufficiently produced in
colliders by virtue of its gauge interactions, one can have interesting
signatures like displaced vertex or disappearing charged tracks provided that
the mass splitting $\delta M$ between DM and the next to lightest stable
particle (NLSP) is small. In particular, if $\delta M < m_\tau=1.77$ GeV, then
we get large displaced vertex signature of NLSP while being consistent with
neutrino mass, lepton flavour violation and observed relic density.
