Since I myself have searched far too long to correctly set my AP addresses at hetzner I wanted to give a small guide for it here.
First you have to change some settings in /etc/network/interfaces. (My config looks like this)
auto lo
iface lo inet loopback
# device: enp5s0
auto enp5s0
iface enp5s0 inet static
address <Host IPv4>
netmask 255.255.255.255
pointopoint <Host gateway>
gateway <Host gateway>
iface enp5s0 inet6 static
address <Host IPv6>2 # like 2001:db8::2
netmask 128
gateway fe80::1
up sysctl -p
auto vmbr0
iface vmbr0 inet static
address <Host IPv4>
netmask 255.255.255.255
bridge_ports none
bridge_stp off
bridge_fd 0
sysctl -w net.ipv4.ip_forward=1
sysctl -w net.ipv6.conf.all.forwarding=1
up ip route add <Additional single IP 1>/32 dev vmbr0
up ip route add <Additional single IP 2>/32 dev vmbr0
up ip route add <Additional single IP 3>/32 dev vmbr0
up ip route add <Additional single IP 4>/32 dev vmbr0
up ip route add <Additional single IP 5>/32 dev vmbr0
up ip route add <Additional single IP 6>/32 dev vmbr0
iface vmbr0 inet6 static
address <Host IPv6>1 # like 2001:db8::1
netmask 64
Host IPv4 = is the lowest ip address without the extra symbol next to it
Host gateway = is the ip under gateway when hovering over the ip address
With the ip address you have to be careful that you change the end because otherwise this does not work like in the example my ips are "0000:0000:0000:49b::2" and "0000:0000:0000:49b::1".
In proxmox for the VM via cloudinit you have to specify this:
just keep in mind that the gateway for the Additional single IP from now on will be the main ips you specified for vmbr0.