  Supernova remnant (SNR) detection along the Galactic plane poses a number of
challenges. The SNR G309.8+00.0 lies exactly on the Galactic plane, with its
center coinciding with galactic latitude (b)=0 deg. In this paper we report the
first detection of the SNR G309.8+00.0 in X-rays and $\gamma$ rays, using
stacked data from the first four consecutive extended ROentgen Survey Imaging
Telescope Array (eROSITA) -- on board the Russian-German Spektrum Roentgen
Gamma (SRG) -- all-sky surveys (eRASS:4) and $\sim15.5$ yr of Pass 8 data
recorded from Fermi-LAT, respectively. The SNR appears to have an elliptical
shape of 0.43 x 0.32 deg in size in both radio synchrotron and X-ray data. The
SNR's emission exhibits a shell-like morphology and good spatial correlation in
both energy bands. The X-ray emission was solely detected in the 1-2 keV energy
band (subject to strong absorption at soft X-rays) and the spectral analysis
results of eRASS:4 data present a purely thermal SNR with a high absorption
column density $3.1_{-0.5}^{+0.7}\cdot10^{22}~\mathrm{cm^{-2}}$ and a
temperature of $0.34\pm0.1$ keV. In combination with optical extinction data,
the absorption column density values derived from the remnant's spectral
analysis support a remnant's distance greater than 6 kpc, rather than a 3.12
kpc distance as reported in the literature, and yield an age of
$1-3.5\cdot10^5$ yr. Employing $\sim15.5$ yr of Fermi-LAT $\gamma$-ray data at
and around the remnant's vicinity, we confirm the detection of the to-date
unidentified 4FGL J1349.5-6206c source that can either be modeled as a single
source or a conglomerate of multiple distinct source components and we argue
that the SNR G309.8+00.0 likely represents at least a significant portion (if
not all) of the emission from the 4FGL J1349.5-6206c $\gamma$-ray source.
