It's too good to be the truth. I think that the board have only gigabit phy (something like realtek RTL8211E - the most cheap rgmii-only phy) connected to port 5 (or port 4).
So probable laytout:
+-------------------+
| MT7620A CPU |
+-------------------+
|
6
+--------------------------------+
| MT7620A |
| embedded switch |
+--------------------------------+
0 1 2 3 5 (RGMII)
| | | | |
LAN1 LAN2 LAN3 LAN4 |
| (RGMII)
+-------------+
| RTL8211E |
+-------------+
|
WAN