Thank you very much @clickingbuttons. It looks like the answer if the higher speed promo was a permanent feature. Given that it expires at the end of the month I will live with what I currently have.The splitter/adaptor you need is a router than can handle 1gbit connections on the WAN and LAN ports.
This will probably work for your use case:
EdgeRouter X - Router - Router - WLAN - 1 Gbps - Amount of ports: - IPSec - IPv6 - L2TP - PPTP - Ethernet - Power over Ethernet - RJ-45 - Managedwww.senetic.co.za