Switch to Genesis DLC

Log into the LOW.MS Control Panel
Go to your ARK server.
Check your server is up-to-date by clicking the Steam Update icon.
Click the Commandline Manager icon.
Unless you are using a custom commandline, re-select your currently select commandline (if you are not sure which, just select the first one)
Change the map name from TheIsland to Genesis
Click Save.
Restart/Start up your server and enjoy! (Make sure you have locally installed the DLC too!)

If you are not a LOW.MS customer, why not avoid the pain of self-hosting by renting an instantly available ARK Genesis server? https://low.ms/game-servers/ark-genesis-server-hosting