Apt update can't resolve repositories

I’m having trouble updating packages in Ubuntu and Centos containers. I’m very new to LXC/LXD and have installed containers to a working state. But after a recent restart I started having trouble connecting to port80

(main host ubuntu18.04) ==> haproxy (container) ==> webserver (container)

The main system is connected. And when installing I was able to install packages and update in all containers. I’ve had some issues with a Ha-proxy container after a restart. It seemed the firewall was blocking port 80. I flushed the firewall rules on the host (Ubuntu18.04).

From what I read on the forums a restart of the system or LXD should rebuild the rules.

However after this I’m not able to update or install any packages on the containers.
I get the following error until all mirrors are exhausted:

$yum update    
http://mirrors.uav.ro/centos/7.7.1908/updates/x86_64/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: mirrors.uav.ro; Unknown error"
    Trying other mirror.

I’m not sure how DNS works in a LXC environment yet.

Anyone know how to troubleshoot & fix this one?