I've been able to dump the nand of this GPON Router and build OpenWrt+custom uboot for it based on HGG420N works by msdos3. However, I can't get ethernet to work, which seems to be a shared issue with Actiontec MI424WR I. They also have similar hardware and both have a QSGMII Module, unlike ZTE F660 and HGG420N. What's really strange is that the stock bootloader doesn't seem to have ethernet working too (I will test it with other parameters later after posting this and share any findings). Any help is appreciated.
UART pinout if anyone needs it (3.3v 115200 8N1 no hardware/software flow control):
Stock firmware CLI is "cisco-like", type "enable" to be able to use the cli, and "help" to list the commands or "system/shell" to get a root shell.