  High contrast imaging enables the determination of orbital parameters for
substellar companions (planets, brown dwarfs) from the observed relative
astrometry and the estimation of model and age-dependent masses from their
observed magnitudes or spectra. Combining astrometric positions with radial
velocity gives direct constraints on the orbit and on the dynamical masses of
companions. A brown dwarf was discovered with the VLT/SPHERE instrument in
2017, which orbits at $\sim$ 11 au around HD 206893. Its mass was estimated
between 12 and 50 $M_{Jup}$ from evolutionary models and its photometry.
However, given the significant uncertainty on the age of the system and the
peculiar spectrophotometric properties of the companion, this mass is not well
constrained. We aim at constraining the orbit and dynamical mass of HD 206893
B. We combined radial velocity data obtained with HARPS spectra and astrometric
data obtained with the high contrast imaging VLT/SPHERE and VLT/NaCo
instruments, with a time baseline less than three years. We then combined those
data with astrometry data obtained by Hipparcos and Gaia with a time baseline
of 24 years. We used a MCMC approach to estimate the orbital parameters and
dynamical mass of the brown dwarf from those data. We infer a period between 21
and 33{\deg} and an inclination in the range 20-41{\deg} from pole-on from HD
206893 B relative astrometry. The RV data show a significant RV drift over 1.6
yrs. We show that HD 206893 B cannot be the source of this observed RV drift as
it would lead to a dynamical mass inconsistent with its photometry and spectra
and with Hipparcos and Gaia data. An additional inner (semimajor axis in the
range 1.4-2.6 au) and massive ($\sim$ 15 $M_{Jup}$) companion is needed to
explain the RV drift, which is compatible with the available astrometric data
of the star, as well as with the VLT/SPHERE and VLT/NaCo nondetection.
