  We develop a pipeline to set new constraints on scale-independent modified
gravity, from the galaxy power spectrum in redshift space of BOSS DR12. The
latter is modelled using the effective field theory of large-scale structure up
to 1-loop order in perturbation theory. We test our pipeline on synthetic and
simulated data, to assess systematic biases on the inferred cosmological
parameters due to marginalization and theoretical errors, and we apply it to
the normal branch of the DGP model with a $\Lambda$CDM background. When applied
to synthetic data and cosmological simulations, we observe biased posteriors
due to the strong degeneracy between the nDGP parameter $\Omega_{\rm rc}$ and
the primordial amplitude of fluctuations $A_s$. Fixing the latter to the Planck
central value, we obtain a posterior distribution with $\Omega_{\rm rc}\lesssim
0.65$ at 95$\%$ C.L., under the assumption of a flat prior on $\log_{10}
\Omega_{\rm rc}$. This upper bound, however, depends strongly on the prior on
$\Omega_{\rm rc}$. To alleviate this effect, we provide an upper bound based on
the Bayes factor between the nDGP model and $\Lambda$CDM model, which gives
$\Omega_{\rm rc}\lesssim 0.2$ at 95$\%$ C.L..
