Yes, essentially it amounts to connecting the voltage of the Power supply to the two couples of wires of the rj45, but with a couple of added features:
1) the connection goes through an electronic on/off switch that can be piloted by the OS/software manually or through scripting (often useful)
2) the amounts of current Is limited to a given amount (It depends on the device model) which may represent a limitation when compared to a PoE injector
I presume that - due to the additional circuitry the power goes through - It Is probably better filtered/leveled but cannot say for sure.
In your specific case, the ONU has low watt needs so you can use a common splitter and converter capable of converting the 24V to the 12V, they are (were) frequently used to Power cameras that are not PoE powered, usually they can provide 1-1.5 A, so 12-18 W at 12 V, though you need to make sure they are "mode B" like the Mikrotiks use.(Pinout 4,5+ 7,8-) as nowadays many are "mode A" AND that they are compatible with passive PoE as nowadays most are 802.3af compatible and they might not work with passive. (the issue Is that - roughly - passive and Mode B are contemporary with 10/100 MB while Active 802.3af/at are contemporary with 1GB, and - even if in theory 802.3af/at should be compatible with both modes, they tend to use Mode A only).
Only as an example, something *like* this:
https://www.aerial.net/shop/product/22_ ... a-out.html
BUT that one has the Power wires not connected, so It won't be 1 GB compatible.
(NO good, but at least the characteristics and limitations are clearly exposed).
I wouldn't be surprised if at the end of the day It would be easier to use a passive splitter provided that you can find a 1 GB one mode B (PoeTexas should have a multi- mode One) and a separate 24v to 12v converter.
If It was not clear from the above, PoE Is a mess in practice.