The 192.168.100.0 networking range is the Starlink router uses to communicate between itself and the Peplink. 192.168.100.1 is the IP address of the Starlink router itself and is how the app you have on your phone communicates with it.
By setting a management address of 192.168.100.100 on the Peplink, that allows the Peplink to route the traffic from the Starlink app on your phone so you can manage it.
Sounds like you might have other configuration or network issues. The health checks and management setting is something I’ve used on hundreds of setups without issues.