Let's find the inflection points, we'll need to solve the equation for this
$$\frac{d^{2}}{d x^{2}} f{\left(x \right)} = 0$$
(the second derivative equals zero),
the roots of this equation will be the inflection points for the specified function graph:
$$\frac{d^{2}}{d x^{2}} f{\left(x \right)} = $$
the second derivative$$2 \left(2 \left(\tan^{2}{\left(x + 1 \right)} + 1\right) \tan{\left(x + 1 \right)} + 9 \left(\tan^{2}{\left(3 x - 1 \right)} + 1\right) \tan{\left(3 x - 1 \right)}\right) = 0$$
Solve this equationSolutions are not found,
maybe, the function has no inflections