OK so in that case you should get LXD to use the legacy xtables driver (iptables, ip6tables and ebtables) as this supports fixing checksums from older DHCP clients.
To do this you need to make sure you have iptables, ip6tables and ebtables installed and that they are not the nftable shim versions (running each command with --version and checking they don’t contain mentions of “nft” should do). See nftables - Debian Wiki
Then you need to clear your nftables ruleset using sudo nft flush ruleset (assuming you’re happy to lose those rules).
Then indicate to LXD that xtables is in use by adding a 1 or more iptables rules, e.g.
sudo iptables -A INPUT
Then reload LXD:
sudo systemctl reload snap.lxd.daemon