Using the substitution
$u'(x)=v(u(x))$ which results in
$u''(x)=v'(u)u'(x)$ or
$u''(x)=v'(u)v(u)$. Plugging these in the ode result in a first order nonlinear ode in
$v(u)$ which is
$v'(u)+\frac{u}{v(u)}-\frac{u^3}{v(u)}=0$ which can now be solved for
$v(u)$. Now use this solution to solve for the first oder order ode
$u'(x)=v(u(x))$.
The problem is with the boundary conditions at
$\pm \infty$. btw, you had them reversed.
I had to sneek in a simpler boundary conditions at zero.
The solution is in terms of inverse function with logs, which you can try to convert to trig. But I plotted it and it does match the
$\frac{\tanh(x)}{\sqrt{2}}$. I only used one of the 2 solutions here.
Clear[u, v, x];
eq = v'[u] + u/v[u] - u^3/v[u] == 0;
sol = v[u] /. First@DSolve[{eq, v[0] == 1/Sqrt[2]}, v[u], u];
sol = u[x] /. First@DSolve[{u'[x] == (sol /. u -> u[x]), u[0] == 0}, u[x], x]
Plotting it side-by-side to the proposed solution shows it is the same
Row[{
Plot[sol /. x -> t, {t, -Pi, Pi}, ImageSize -> Medium, PlotLabel -> "DSolve"],
Plot[Tanh[x/Sqrt[2]], {x, -Pi, Pi}, ImageSize -> Medium, PlotLabel -> "Given solution"]}
]
