The local/internal IP of the Proxidize server is assigned by the router automatically once the device is connected to the router via ethernet.
Communication between the Proxidize server and the router is done through the Dynamic Host Configuration Protocol (DHCP). This process applies to any device connected to the router via ethernet or Wifi.
The server is assigned an IP address with a specific lease time set by default by the router. This means the IP will be withdrawn after this lease expires or if your Proxidize server is rebooted.
This withdrawal will force you to edit your port forwarding rules every time this happens, or you'll need to identify the local IP of your server every time you want to access it internally.
To mitigate this, you must reserve the IP statically in the router. Increasing the lease time will help temporarily, but you'll go through the issue again once the lease time is over.
The steps can vary from router to router, but the concept is the same for all routers.
Log in to the router.
Look for DHCP Settings. If you use the recommended TP-Link AC1200, go to Advanced > Network > DHCP Server.
Click on "Add Reservation" > Add the IP address of your Proxidize server.
Once the configurations are saved, the IP address of your Proxidize will be reserved statically and won't change dynamically after the lease time has expired or after the server reboots.