There is MAC-spoofing issue with official firmwares: when device just powered up (after u-boot?) its works like unmanaged 5-port switch i.e. sending all your lan MACs to WAN port which could cause port ban in case of some ISPs
IIRC in *wrt fimwares there is some register modificaton performed at linux boot stage to prevent this bug. While vendor provides fix at earlier u-boot stage i.e. with modified bootloader
With new vendor boot code WAN port getting disabled at device-startup. Then original firmwares restores WAN port link while *wrt-based doesn't.
here is beta-firmware:
link
have also old one with original bootloader but can't upload there (4Mb upload limit?)