I am assuming the CPU for your board, MT7621DAT is the same as MT7621A
Here is the commit for a very similar board, start studying the DTSI file to understand how the kernel is configured in this situation. (in this commit they converted a DTS into a common code DTSI which is included for more than one board).
probably you need a UART cable to access bootloader and root shell
start collecting info such as:
what bootloader there is and the options for booting
kernel log
flash partitions cat /proc/mtd
dumping the "factory" partition hexdump -C /dev/mtdX