Does your router indicate that you have DS-Light? I think O2 provides each customer DS-Light until they ask for a real IPv4.

To your second question: In case of DS-Light you don’t need a new IPv4 IP every 24h because your IP is not public facing.

PS: I don’t be sure, but the Fritz Remote Apps use IPv6 to ensure that they also work with DS-Light.

Yes I use wireguard only with systemd-networkd (as server and as client).

I followed the arch wiki and you need to ensure that the file permissions are correct, otherwise systemd will ignore them.

Copied from the wiki:

# chown root:systemd-network /etc/systemd/network/99-*.netdev # chmod 0640 /etc/systemd/network/99-*.netdev