How to start the VPS network card settings correctly?

Question

ubuntu

3rd day I suffer with IPV6
Hetzner suggested not to use the DHCP network card and assign resources manually. OK.
We write:

auto ens3
iface ens3 inet static
        address <ваш IPv4 адрес>
        netmask 255.255.255.255
        gateway 172.31.1.1
        pointopoint 172.31.1.1
        dns-nameservers 213.133.98.98 213.133.99.99 213.133.100.100

iface ens3 inet6 static
        address <один IPv6 адрес из вашей подсети, например, 2001:db8:0:3df1::1>
        netmask 64
        gateway fe80::1

As a result, the bolt, the network does not rise - gives an error (in the syslog that the unit fell and did not wring out, no specifics, what for then these logs ....)

If we use it like now:

auto ens3
iface ens3 inet dhcp

That is no problem, everything is clear - the only thing is not registered gateway fe80::1 для IPV6

Where to dig?
I’m already afraid to experiment too much, because I’ll lose access to the server even through the Admin panel console, then write wasted ..