Two domains, two purposes
The two domains are used for two distinct purposes, which is why we suggest setting both up.
Your custom control panel URL is what you or your clients will see when you log in to MyWiFi. This means that your clients will never need to see MyWiFi at any point.
In contrast, your custom social portal URL is what your clients' guests will see when they use MyWiFi to authenticate and log in to the WiFi. This means that the guests also do not need to see MyWiFi at any point.
You need to use different subdomains for each of these custom URLs, because they need to point to different places. We generally suggest something like dashboard.yourdomain.com for your control panel URL and social.yourdomain.com for your social portal URL so that you can differentiate.