Port forwarding while on local LAN

I have a mobile app that send MQTT messages to a DDNS name which resolves to the DPC3941B modem.  A port forwarding rule sends the traffic to a server on the local LAN.  Works great until the mobile device gets on wifi.  DNS resolves to the same IP address, but the port fordwarding no longer works onceit's on the local LAN.

I found this - https://business.comcast.com/help-and-support/internet/comcast-business-ip-gateway-static-firewall/#portforward

• Local computers (devices on the same subnet) must access the local server using the computers’ local LAN address (10.1.10.x, by default) instead of the public IP used to access it from external connections. Attempts by local computers to access the server using the external WAN IP address will fail.

Is this really the case?  I can't have a mobile app connect from both the internet and from the local LAN using the same DDNS public IP?