Hello from SB dhoomakethu. 
This looks mighty similar to the resolver regression on ubuntu-core which is about test failures with the same DNS failure.
This workaround works for me: I found a suggestion on another forum which seems to partly solve the problem
So now if I run : sudo systemctl restart snapd.service And then snap refresh
The snap refresh works ok. However it is not a permanent fix as if I wanted to install a new snap I need to run sudo systemctl restart snapd.servcice again before each new install/update.
Again if I leave my unit overnight the automatic update seems to fix the issue.
Here are my results from a DELL-5000 fresh out of the box:
admin@localhost:~$ snap list
Name Version Rev Developer Notes
bluez 5.44-3 129 canonical -
ccm-wda 3.01.13.40419 18 dell-inc -
core 16-2.31.2 4206 canonical core
modem-manager 1.6.12-1 120 canonical -
network-manager 1.2.2-14 250 canonical -
snapweb 0.26-11 319 canonical -
stlouis 16.04-1.14 13 canonical gadget
stlouis-kernel 4.4.0-116.140 38 canonical kernel
tpm2 1.0-5 42 canonical -
udisks2 2.6.4-2 100 canonical -
uefi-fw-tools 1.5.2-0.7.2+git 10 canonical -
wifi-ap 17 204 canonical -
admin@localhost:~$ snap refresh
error: cannot refresh: cannot refresh snap-declaration for "stlouis": Get
https://api.snapcraft.io/api/v1/snaps/assertions/snap-declaration/16/Vy4C9zwJOHQRpmGpUNxxwBqdI63Oj6lM?max-format=2:
dial tcp: lookup api.snapcraft.io on [::1]:53: read udp
[::1]:38943->[::1]:53: read: connection refused
admin@localhost:~$ sudo systemctl restart snapd
admin@localhost:~$ snap refresh
Setup snap "core" (6405) security profiles O
Broadcast message from root@localhost.localdomain (Fri 2019-02-15 00:29:42 UTC):
reboot scheduled to update the system - temporarily cancel with 'sudo shutdown -c'
The system is going down for reboot at Fri 2019-02-15 00:39:41 UTC!
This is definitely a bug, not a network configuration error. Either in the resolver, snapd or the core image / install on the DELL-(35)000.