I am trying to solve $$ \left(\eta ^2-1\right) \cosh ^2(\beta \eta )+2 J^2=2 \eta (J \sinh (\beta \eta ) \sinh (\beta J)+\eta \cosh (\beta \eta ) \cosh (\beta J)) $$ for $\eta$. Where, $\beta, J$ are positive and reals, and $B$ is real. I used
Reduce[2 J^2 + (η^2 - 1) Cosh[ β η]^2 == 2 η (η Cosh[J β] Cosh[β η] + J Sinh[J β] Sinh[β η]) && η > 0 && J > 0 && β > 0, η, Reals]
with no answers.