Finally, I made the XDR 608x works. I made the LEDs of the two 2.5G ports lights and made them works at a speed of less than 2.5Gbps (I've tested 2.5G/1G/100M Full Duplex) without packet loss.
You can find my source code at https://github.com/cyyself/openwrt/tree/tl-xdr608x and compile it yourself.
There is some further work to make it available to be upstream.
Since we don't have a step-by-step tutorial just now, I strongly recommend you have a TTL cable connected before flashing the router and back up the entire flash (which contains the wifi firmware set by the factory and it's not identical for each device) before writing the OpenWRT. The U-Boot will show some important messages through TTL UART, don't let U-Boot runs the default boot command the first time. I have broken my router once, the U-Boot is even broken, and then saved it by the CH341 programmer and WSON8 probe. It takes me about 100 CNY and a few days to wait for the package.