At 5:07, there is a closeup of a RTL8197.
At 5:01 you can see a Row of 4 Pinpads along the front edge of the PCB. 3 of those will likely be the UART.
Maybe also check out (the end of) Any plans for Realtek SOC support? - #78 by G-Shtuller , and/or attempt to build from https://github.com/ggbruno/openwrt