This Xiaomi ax3600 ssh guide claims to add presistent telnet/ ssh/ uart access to the device, by changing the nvram defaults hardcoded in the bdata partition. This procedure has been confirmed by multiple users for the chinese firmware, given the way it is implemented I'd assume it would also work on the international variant, but obviously that's not guaranteed (but given that it seems to be possible to change between both variants at will, this shouldn't be that much of an issue).