  Walker-Wang models are fixed-point models of topological order in $3+1$
dimensions constructed from a braided fusion category. For a modular input
category $\mathcal M$, the model itself is invertible and is believed to be in
a trivial topological phase, whereas its standard boundary is supposed to
represent a $2+1$-dimensional chiral phase. In this work we explicitly show
triviality of the model by constructing an invertible domain wall to vacuum as
well as a disentangling generalized local unitary circuit in the case where
$\mathcal M$ is a Drinfeld center. Moreover, we show that if we allow for
fermionic (auxiliary) degrees of freedom inside the disentangling domain wall
or circuit, the model becomes trivial for a larger class of modular fusion
categories, namely those in the Witt classes generated by the Ising UMTC. In
the appendices, we also discuss general (non-invertible) boundaries of general
Walker-Wang models and describe a simple axiomatization of extended TQFT in
terms of tensors.
