For example, i need to remote access to a new ePMP1000 AP behind a router, from which i can ping them, but not access because the default gateway is not set in the AP.
The “gateway” IP address will be a router on your network, that you will have assigned and configured. This will be different on different customers networks.
Therefore, when a unit is set to factory default a default IP address and subnet is given. You must then connect to the device the from the same local network as this default IP address - you will need to configure your PC to be another device on this network. You can then program the unit with your own networks IP details, including the device IP address, subnet mask and your networks default gateway.
Details on how the default settings, and how to configure the IP settings are given in the user manual.