Hi Mario, after Corey has given some input, we can try finding out what happens on your Pi. Does Raspbian use Network-Manager? systemd-networkd? ifupdown?
You can try adding After=network-online.target Wants=network-online.target to the [Unit] stanza of ser2net.service. According to https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/ you need to enable the correct wait-online.service matching your network management software (NetworkManager, systemd-networkd, ifupdown etc) for this to reliably work. Greetings Marc
