  We derive new constraints on the CIB monopole spectrum from reprocessed
COBE-DIRBE sky maps with lower instrumental and astrophysical contamination
than the legacy DIRBE maps. These maps have been generated through a global
Bayesian analysis framework that simultaneously fits cosmological,
astrophysical, and instrumental parameters, described in a series of papers
referred to as Cosmoglobe DR2. We have applied this method to the DIRBE
Calibrated Individual Observations, complemented by selected HFI and FIRAS sky
maps to break key astrophysical degeneracies, as well as WISE and Gaia compact
object catalogs. In this paper, we focus on CIB constraints that result from
this work. We report detections of an isotropic signal in five out of the ten
DIRBE bands (1.25, 2.2, 3.5, 140, and 240 $\mu$m). For the 2.2 $\mu$m channel,
we find an amplitude of $13\pm3\,\mathrm{nW\,m^{-2}\,sr^{-1}}$, 36% lower than
reported from the official DIRBE release. For the 240 $\mu$m channel, we find
$10\pm3\,\mathrm{nW\,m^{-2}\,sr^{-1}}$, 26% lower than the official DIRBE
release. We interpret these lower values as resulting from improved zodiacal
light and Galactic foreground modeling. For the bands between 4.9 and 100
$\mu$m, the presence of excess radiation in solar-centric coordinates reported
in a companion paper precludes the definition of lower limits. However, the
analysis still provides well-defined upper limits. For the 12 $\mu$m channel,
we find an upper 95% confidence limit of 103 $\mathrm{nW\,m^{-2}\,sr^{-1}}$, a
factor of four lower than the corresponding legacy result of 468
$\mathrm{nW\,m^{-2}\,sr^{-1}}$. The results presented in this paper redefines
the state-of-the-art CIB monopole constraints from COBE-DIRBE, and it provides
a real-world illustration of the power of global end-to-end analysis of
multiple complementary data sets which is the foundational idea of the
Cosmoglobe project.
