How can I add a route that is "on the link", directly connected to the interface even if the IPs are not the same?
Starting with netplan 0.34 (now in Ubuntu 18.04), you can add "scope: link" to your route in netplan config, with something like this:
network:
version: 2
renderer: networkd
ethernets:
eth0:
[...]
gateway4: 62.210.0.1
routes:
- to: 62.210.0.1/32
via: 62.210.0.1
scope: link