networking – netplan: routes and different IP , how to configure it?

I spent the whole morning trying to solve this.

I have two IP addresses available on the same interface (different gateways), let’s say 100.100.100.100 and 200.200.200.200.

My YAML is:

network:
  version: 2
  renderer: networkd
  ethernets:
    eth0:
      match:
        macaddress: u.u.u.u
      addresses:
        - 100.100.100.100/32
        - 200.200.200.200/32
      #gateway6: fe80::1
      routes:
        - to: 0.0.0.0/0
          via: 100.100.100.1
          on-link: true
      nameservers:
        search: ( invalid )
        addresses:
          - x.x.x.x
          - y.y.y.y

I tought that, to let this server surf the net with 200.200.200.200, it would be easy as changing the “via” line to:

via: 200.200.200.1

This doesn’t work. I launch netplan apply and nothing changes, I always surf the web with 100.100.100.100 (as stated by curl icanhazip.com and also in ifconfig).

What am I missing?