Well, it is a MT7621 device with MT7615 WiFi chips. I know MT7615 are not open source some I don't care about the WiFI right now (even though they are still plugged in on the device).
So, I've compiled an OpenWrt build for MT7621 with very basic packages so I could just boot it. However, I couldn't boot it and I don't know how to debug it.
I've flash this firmware using tftp, which after some work, I got it to upload it to the hardware. But now I'm stuck. Any light and help is appreciated.
This may require disassembling the flash and burning the firmware using the burner.
To say something off-topic, even if you just use the mt7621 device as a wired router, its experience is quite bad. Usually you can't use dhcp to get ip or encounter compatibility problems between Gigabit switch chip and client Gigabit LAN.