You don't have to configure your modem to use 121Co or share Wifi.
The current configuration of your modem is sufficient.
As a host, you will only be sharing your Wifi access details with the sharer similar to how you would share your Wifi details with a family member or guest who visits you.
As a sharer, you don't have to do any configuration as you will be sharing your host's modem. The host will provide you with Wifi access details.