GL-B1300 use external switch chip QCA8075.
IPQ4018 connect with QCA8075 by SGMII physically.
port 0 is cpu port which connect the whole switch.
port 1-5 is the port related to the actual physical port.
port 5 is wan while port 1-4 is lan, they are seperated
by port-based vlan.

Here you can check gmac0/eth0 has vid 2 and port map 0x20, which only indicate the port 5.
while gmac1/eth1 has vid 1 and port map 0x1e.