I have one with MT7620+MT7610E, 8+64MB, which is the same as ASUS RT-AC51U except for lower flash capacity. At least 2 third-party firmwares can be flashed:
https://openwrt.org/toh/asus/rt-ac51u
https://bitbucket.org/padavan/rt-n56u/downloads/ (use the "base" firmware for 8MB flash)
I'm using the latter one since the former one has an issue on 2.4G channel (openwrt 23.05, 2.4G signal disappears serval hours after booting, a hardware reboot is required).
You might need a USB-serial cable and upload firmware via tftp, or use an spi flash programmer to flash a bootloader called breed. I have flashed breed-mt7620-reset12.bin.