Hi Frank,

I followed above steps and did not install any pakcages, just keep it as a simple server.
But got the same result, after the reboot, lost the route info.
And sudo netplan apply --dryrun did not pop any prblems,

This is some runlog.

Welcome to Ubuntu 22.04.1 LTS (GNU/Linux 5.15.0-52-generic s390x)
 * Documentation:  https://help.ubuntu.com
 * Management:     https://landscape.canonical.com
 * Support:        https://ubuntu.com/advantage
  System information as of Fri Nov 11 02:53:35 AM UTC 2022
  System load: 0.02783203125     Memory usage: 6%   Processes:       538
  Usage of /:  9.0% of 15.60GB   Swap usage:   0%   Users logged in: 0
41 updates can be applied immediately.
To see these additional updates run: apt list --upgradable
Failed to connect to https://changelogs.ubuntu.com/meta-release-lts. Check your Internet connection or proxy settings
Last login: Fri Nov 11 02:50:21 UTC 2022 from 10.20.92.70 on pts/0
[?2004hubuntu@a90ubtu24:~$ 
ping -c 5 10.20.103.64
[?2004l
ping: connect: Network is unreachable
[?2004hubuntu@a90ubtu24:~$ 
route
[?2004l
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
[?2004hubuntu@a90ubtu24:~$ 
ip addr
[?2004l
1: lo:  mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: enc1000:  mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 02:ca:72:80:04:39 brd ff:ff:ff:ff:ff:ff
3: enc1000.1300@enc1000:  mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 02:ca:72:80:04:39 brd ff:ff:ff:ff:ff:ff
    inet 10.20.103.69/24 brd 10.20.103.255 scope global enc1000.1300
       valid_lft forever preferred_lft forever
[?2004hubuntu@a90ubtu24:~$ 
sudo netplan apply --dryrun
[?2004l
[sudo] password for ubuntu: 
[?2004hubuntu@a90ubtu24:~$ 
sudo netplan apply
[?2004l
[?2004hubuntu@a90ubtu24:~$ 
route
[?2004l
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         _gateway        0.0.0.0         UG    0      0        0 enc1000.1300
10.20.103.0     0.0.0.0         255.255.255.0   U     0      0        0 enc1000.1300
[?2004hubuntu@a90ubtu24:~$ 
ip addr
[?2004l
1: lo:  mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: enc1000:  mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 02:ca:72:80:04:39 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::ca:72ff:fe80:439/64 scope link 
       valid_lft forever preferred_lft forever
3: enc1000.1300@enc1000:  mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 02:ca:72:80:04:39 brd ff:ff:ff:ff:ff:ff
    inet 10.20.103.69/24 brd 10.20.103.255 scope global enc1000.1300
       valid_lft forever preferred_lft forever
    inet6 fe80::ca:72ff:fe80:439/64 scope link 
       valid_lft forever preferred_lft forever
[?2004hubuntu@a90ubtu24:~$ 
ping -c 5 10.20.103.64
[?2004l
PING 10.20.103.64 (10.20.103.64) 56(84) bytes of data.
64 bytes from 10.20.103.64: icmp_seq=1 ttl=0 time=0.817 ms
64 bytes from 10.20.103.64: icmp_seq=2 ttl=0 time=1.13 ms
64 bytes from 10.20.103.64: icmp_seq=3 ttl=0 time=0.258 ms
64 bytes from 10.20.103.64: icmp_seq=4 ttl=0 time=0.284 ms
64 bytes from 10.20.103.64: icmp_seq=5 ttl=0 time=0.281 ms
--- 10.20.103.64 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4195ms
rtt min/avg/max/mdev = 0.258/0.554/1.133/0.357 ms
[?2004hubuntu@a90ubtu24:~