When a Proxmox server connects to a new network (e.g. a new router), its network configuration needs to be changed.

Edit /etc/network/interfaces and /etc/hosts to change IP address and gateway, /etc/resolv.conf to change DNS. Then reboot the system.

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# Use nano to edit
nano /etc/{network/interfaces,hosts,resolv.conf}
# Use Vim to edit
vim -p /etc/{network/interfaces,hosts,resolv.conf}
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reboot