Comment 3 for bug 1765341

> your subnet does not contain the gateway address

Yes.

> pointtopoint configurations aren't supported by the default server installer

That the issue.

I don't use netplan, but use systemd-network directly, my config is:
https://nixtux.ru/506

/etc/systemd/network/50-1vds.network

[Match]
Name=ens*
[Network]
DHCP=no
# ping to Google DNS 8.8.8.8 is less than to Yandex 77.88.8.*
DNS=8.8.8.8
DNS=77.88.8.8
DNS=77.88.8.1
DNS=9.9.9.9
NTP=ntp.ubuntu.com
Gateway=10.0.0.1
[Address]
Address=212.109.197.147/32
Peer=10.0.0.1