The MT7621 external RGMII interface is an infinite source of confusion. You need to figure out if the phy is supposed to be connected to the switch or the 2nd gmac. It seems to me that you want the latter, but I could be wrong there. I still have problems bending my head around this.
See the great explanation and pointer @Thirsty gave here: Which registers to use to enable RGMII2 on the MT7621 - #3 by telecom21