Default gateway change is effective only after reboot

[kamild1996]

Description of issue/question

When you change the default gateway of a network interface in OMV's network settings, the new gateway is being added but it's not set as a default gateway, rendering the change ineffective. Rebooting the device removes the old gateway, which finally makes the change effective.
Result of the sudo ip route command after changing the gateway (in this case, 10.0.0.128 is the old gateway and 10.0.0.1 is the new one):

default via 10.0.0.128 dev eth0 proto static
default via 10.0.0.1 dev eth0 proto static
10.0.0.0/24 dev eth0 proto kernel scope link src 10.0.0.20
172.16.24.0/24 dev ztrfyhcrsm proto kernel scope link src 172.16.24.1
172.17.0.0/16 dev docker0 proto kernel scope link src 172.17.0.1 linkdown

Steps to reproduce issue

1. Login to your OMV control panel
2. Go to "System > Network", to the "Interfaces" tab.
3. Change the gateway and save changes.
4. Verify with sudo ip route that the new gateway was added, but not set to default.

Versions report

5.3.0-1 (Usul)

[votdev]

OMV uses systemd to configure the network. If systemd can't handle this, then we can not do anything here. OMV is not able to detect which config option has been changed, so no individual action can be done to workaround this.