  We present the Fermi-LAT observations of the behind-the-limb (BTL) flare of
July 17, 2021 and the joint detection of this flare by STIX onboard Solar
Orbiter. The separation between Earth and the Solar Orbiter was 99.2$^{\circ}$
at 05:00 UT, allowing STIX to have a front view of the flare. The location of
the flare was ~S20E140 in Stonyhurst heliographic coordinates making this the
most distant behind-the-limb flare ever detected in $>$100 MeV gamma-rays. The
LAT detection lasted for $\sim$16 minutes, the peak flux was $ 3.6 \pm 0.8 $
(10$^{-5}$) ph cm$^{-2}$ s$^{-1}$ with a significance $>$15$\sigma$. A coronal
wave was observed from both STEREO-A and SDO in extreme ultraviolet (EUV) with
an onset on the visible disk in coincidence with the LAT onset. A complex type
II radio burst was observed by GLOSS also in coincidence with the onset of the
LAT emission indicating the presence of a shock wave. We discuss the relation
between the time derivative of the EUV wave intensity profile at 193\angstrom\
as observed by STEREO-A and the LAT flux to show that the appearance of the
coronal wave at the visible disk and the acceleration of protons as traced by
the observed $>$100 MeV gamma-ray emission are coupled. We also report how this
coupling is present in the data from 3 other BTL flares detected by Fermi-LAT
suggesting that the protons driving the gamma-ray emission of BTL solar flares
and the coronal wave share a common origin.
