Just a wild guess, but it sounds like Apache is being started before the network is online. Is this host simple static Ethernet or DHCP, or is it something more complicated like WiFi? I believe you can set service dependencies with systemd, ideally your apache service isn't started before the network is online. If you are using WiFi, running a webserver is a bit ugly, perhaps configuring Apache to listen to 0.0.0.0 (all interfaces) would be a better choice?