  The problem of missing Galactic supernova remnants (SNRs) refers to the issue
that the currently known Galactic SNRs are significantly incomplete compared to
the theoretical prediction. To expand the sample of Galactic SNRs, we use GLEAM
and THOR+VGPS data across four wavebands ranging from 118 to 1420 MHz to drive
a spectral index map covering the region within 26.6{\deg} < l < 30.6{\deg},
$\vert b \vert \leq$ 1.25{\deg}, where numerous SNR candidates were recently
found. By using the spectral index map of the sky region and detailed analysis
of the spectral indices of individual sources, we confirmed four SNR
candidates, namely G26.75+0.73, G27.06+0.04, G28.36+0.21, and G28.78$-$0.44, as
SNRs. Additionally, we discovered an expanding molecular superbubble located in
this region, discussed pulsars associated with SNR candidates, and discovered a
long H$\alpha$ filament that spatially overlaps with the candidate G29.38+0.10.
We suggest that the problem of missing Galactic SNRs not only arises from
observation limitations, but also could be due to the low-density environments
of some SNRs, and the different SN explosion properties.
