Hetzner subnets do not work with cloud-init

[Mathiiss]

Hello, my subnet is not working when I create a virtual machine with cloud-init, the connection does not work while when I create an LXC container, the internet connection is working fine.

/etc/network/interfaces

### Hetzner Online GmbH installimage

source /etc/network/interfaces.d/*

auto lo
iface lo inet loopback
iface lo inet6 loopback

auto enp34s0
iface enp34s0 inet static
  address 136.243.XXX.XXX
  netmask 255.255.255.192
  gateway 136.243.138.193
  up route add -net 136.243.XXX.XXX netmask 255.255.255.192 gw 136.243.XXX.XXX dev enp34s0

iface enp34s0 inet6 static
  address 2a01:4f8:212:XXXX::2
  netmask 128
  gateway fe80::1

# VM-Bridge used by Proxmox Guests
auto vmbr0
iface vmbr0 inet static
  # Our Host will be the Gateway for all VM's
  address XXX
  netmask 255.255.255.192
  # Some basic bridge configuration
  bridge_stp off
  bridge_ports none
  bridge_fd 0
  bridge_maxwait 0

  # Add all single IP's from your /29 subnet
  up ip route add 5.9.XXX/28 dev vmbr0

iface vmbr0 inet6 static
  # Our Host again as the Gateway
  address 2a01:4f8:212:XXXX::2
  # Here we use the /64 subnet!
  netmask 64

 

[Mathiiss]

I have tried this workaround (https://forum.proxmox.com/threads/cloud-init-ubuntu-18-04-bug-mit-default-gw.45286/), it works fine, but I have to do it manually which is a problem since I want my VM to configure itself automatically.