This setup shows how to configuure systemd-networkd and pppd to talk to an ISP using Open Eir's infrastructure, which I believe is fairly similar to other countries' setup.
I have two interfaces, which I configure as ext0 (external) and lan0 (internal).
An 802.1q VLAN with tag 10 (vlan10) is set up on the external interface.
A pppoe service is configured to launch PPPoE on this VLAN (ie: pppoe@vlan10.service), wanted by sys-devices-virtual-net-vlan10.device.
systemd-networkd acts as DHCP and RA service on the LAN interface.