They both use MediaTek MT7905DAN n MediaTek MT7975DN as wifi chip n MediaTek MT7621AT at cpu
They both have 128 MiB of flash n 256 MiB of ram
The difference between them is ax53u have 1 x usb port n 3 x lan port n ax1800u have 0 x usb port n 4 x lan port n different LED configuration between them.
becuae of the similarity of them i think openwrt image for ax53u might work on ax 1800hp so i flash the openwrt image for ax53u to ax 1800hp .
Openwrt booted on ax1800hp. it work fine (expect for broken LED configuration n wan port dont work n lan port 4 become the wan port)
Edit : is lan port 4 not lan port 1
Edit2 : i lie the the flash layout r that same between them.
Edit3 : NOTE : openwrt now support ax1800 (ax54) so this hack atin needed (see OpenWrt support for Asus AX1800HP)
I notice it might possible to get the wan port working at a lan port by using older image of openwrt's ax53u image because the device tree of the older image of openwrt's ax53u's drive tree r missconfiged to have 4 x lan n 1 x wan
(yes i know i could rebuild the kernel that have the device tree that configed to have 4xlan 1xwan)