As it turned out, some of the low-end Asus routers are not supported by OpenWrt (and/or LEDE). Also, due to wonders of the marketing department, the boards are shared between some of these, i.e.:
– Asus RT-N12+ == RT-N11P == RT-N12E b1
– Asus RT-N12+ V3 == RT-N300 B1
– Asus RT-N12E c1 == ???
(You can spend some time on wikidevi to map these, use the FCC ID as a confirmation for a match).
These are usually built on a flavour of Ralink MIPS boards (MT7620a/n, MT7628), so can be easily supported with a standard DTS added, the difference is mostly around LED configs, buttons, and USB enabled/disabled.
To cover the first set, a patch was sent as an example:
If you want to add one yourself, files that need changes:
… and copy the DTS in target/linux/ramips/dts from the closest sibling. You can use the “rt-n12p” device target as an example.