Trigger script when the network route changes

Hi. I am looking for a solution to trigger a script when the default route changes/main interface is down/restored.

So, I have 2 network interfaces:

- ethernet enp2s0 with default metric 1

- 4g mobile enx0 with default metric 2

When Ethernet goes down, the metric changes and vice versa. I am getting in systems notifications something like this:

systemd-resolved[628]: enp2s0: Bus client set default route setting: no

systemd-resolved[628]: enx0: Bus client set default route setting: yes

When the Ethernet connection is restored, it changes back:

systemd-resolved[628]: enp2s0: Bus client set default route setting: yes

systemd-resolved[628]: enx0: Bus client set default route setting: no

I have Ubuntu SystemD, and I have self-written script to send any notifications via one of public messengers, so in my case there will be 2 scenarios:

1. send_messenger.sh "me" "the traffic is routed through 4g connection from now, please, be patient" when Ethernet is down and the modem enabled
2. send_messenger.sh "me" "ethernet is back, the normal bandwidth is available" when Ethernet goes back.

What may be a systemd unit service which may trigger that script when the default route changes? Or one of the network interfaces is disconnected?