LXC containers: DNS issues and networking.service failed

[RabbinowitZ]

Hello guys! I'm sorry for my bad english, but it's not my first language.

I have a proxmox 6.3-3 server (community subscription) with 5 VMs and 4 Turnkey Linux CTs (1 privileged, 3 unprivileged).

Every morning I check the systems and every morning the DNS (which is a Pi-Hole in a VM) does not resolve the hostnames of the CTs. If I ping them via IP, I have no problem. Same with SSH and Webmin, no problem via IP. If I reboot the CTs, the issue disappears only to reappear the next day.
I have absolutely no issues with DNS on VMs.

Another problem is the networking.service: it fails on every CTs, even after rebooting them or rebooting Proxmox itself.

systemctl status networking.service

systemctl status networking.service
* networking.service - Raise network interfaces
   Loaded: loaded (/lib/systemd/system/networking.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Mon 2021-01-04 08:31:57 CET; 37min ago
     Docs: man:interfaces(5)
  Process: 8712 ExecStart=/sbin/ifup -a --read-environment (code=exited, status=1/FAILURE)
Main PID: 8712 (code=exited, status=1/FAILURE)

Jan 04 08:31:57 ctostam ifup[8712]: udhcpc: lease of 192.168.1.243 obtained, lease time 86400
Jan 04 08:31:57 ctostam ifup[8712]: /etc/udhcpc/default.script: Resetting default routes
Jan 04 08:31:57 ctostam ifup[8712]: SIOCDELRT: No such process
Jan 04 08:31:57 ctostam ifup[8712]: /etc/udhcpc/default.script: Adding DNS 192.168.1.253
Jan 04 08:31:57 ctostam ifup[8712]: /etc/resolvconf/update.d/libc: Warning: /etc/resolv.conf is not a symbolic link to /etc/resolvconf/run/resolv.conf
Jan 04 08:31:57 ctostam ifup[8712]: No DHCPv6 client software found!
Jan 04 08:31:57 ctostam ifup[8712]: ifup: failed to bring up eth0
Jan 04 08:31:57 ctostam systemd[1]: networking.service: Main process exited, code=exited, status=1/FAILURE
Jan 04 08:31:57 ctostam systemd[1]: networking.service: Failed with result 'exit-code'.
Jan 04 08:31:57 ctostam systemd[1]: Failed to start Raise network interfaces.

systemctl restart networking.service

systemctl restart networking.service
Job for networking.service failed because the control process exited with error code.
See "systemctl status networking.service" and "journalctl -xe" for details.

journalctl -xe

journalctl -xe
Jan 04 09:10:03 ctostam systemd[1]: networking.service: Main process exited, code=exited, status=1/FAILURE
-- Subject: Unit process exited
-- Defined-By: systemd
-- Support: https://www.debian.org/support
--
-- An ExecStart= process belonging to unit networking.service has exited.
--
-- The process' exit code is 'exited' and its exit status is 1.
Jan 04 09:10:03 ctostam systemd[1]: networking.service: Failed with result 'exit-code'.
-- Subject: Unit failed
-- Defined-By: systemd
-- Support: https://www.debian.org/support
--
-- The unit networking.service has entered the 'failed' state with result 'exit-code'.
Jan 04 09:10:03 ctostam systemd[1]: Failed to start Raise network interfaces.
-- Subject: A start job for unit networking.service has failed
-- Defined-By: systemd
-- Support: https://www.debian.org/support
--
-- A start job for unit networking.service has finished with a failure.
--
-- The job identifier is 4129 and the job result is failed.
Jan 04 09:10:05 ctostam stunnel[276]: LOG5[868]: Service [webmin] accepted connection from 192.168.1.121:50060
Jan 04 09:10:05 ctostam stunnel[276]: LOG5[868]: s_connect: connected 127.0.0.1:10000
Jan 04 09:10:05 ctostam stunnel[276]: LOG5[868]: Service [webmin] connected remote server from 127.0.0.1:47944
Jan 04 09:10:06 ctostam stunnel[276]: LOG3[868]: transfer: s_poll_wait: TIMEOUTclose exceeded: closing
Jan 04 09:10:06 ctostam stunnel[276]: LOG5[868]: Connection closed: 639 byte(s) sent to TLS, 679 byte(s) sent to socket
Jan 04 09:10:09 ctostam stunnel[276]: LOG5[869]: Service [webmin] accepted connection from 192.168.1.121:50065
Jan 04 09:10:09 ctostam stunnel[276]: LOG5[869]: s_connect: connected 127.0.0.1:10000
Jan 04 09:10:09 ctostam stunnel[276]: LOG5[869]: Service [webmin] connected remote server from 127.0.0.1:47948

I've been having these issues since I installed the CTs and I guess these two problems are related in some way.
Can someone help me?
Thanks in advance.

Best regards,
Massimo