In the Edgerouter X device wiki page there are instructions to add PoE also to the other ports (eth1..3) of the device. I am reaching out to anyone who has done it or who can answer a few questions about it.
There it says:
remove R137 & R138
connect R137 pad to R131
connect R138 pad to gnd or R131
After removing R137 it leaves two empty pads on the PCB. Which one should be connected to R131? Also, does it matter on which pad of the R131 the R137 is connected to?
What is the direction of PoE that you get after above modifications? Out?
What does the warning "transfomer MAX 600mA" mean in the end of the instructions? I assumed passive PoE means passthrough of externally supplied power but I may be wrong. The vision in my mind is that after modding all the ports eth1..3 I could supply PoE IN through eth0 and the rest of the ports would have power flowing out. But I am expecting that devices in all ports eth1...4 will draw more than 600mA current.
The ER-X only supports 24V passive PoE. Unlike 802.3af (i.e. normal active PoE which will automatically detect if it should be enabled), passive PoE doesn't have any way to determine if PoE should or should not be applied to a port. This means it is always on which can damage devices that are not designed for PoE.
In the EdgeMax firmware (and I assume OpenWrt as well), the PoE output on eth4 can be enabled/disabled (which is critical based on #1 above). It is not clear to me if the mod described would provide true always-on 24V passive PoE, or if it can be enabled/disabled in software (it likely would be either always on for the modded ports, or PoE switchable on all ports at the same time; you would likely not have individual port control).
As mentioned earlier, this is passive 24V PoE only for both the PoE input and output. It will not be possible to power the device from an 802.3af/at/bt power source (unless you use a special adapter that converts it to 24V passive).
Further, the output is not capable of powering devices that require 802.3af/at/bt (which is 48-52V active). Some older Unifi APs (and cameras) use the 24V PoE method, but most of the newer devices cannot.
The power adapter that ships with the ER-X does not have enough capacity to provide PoE output on eth4, let alone multiple ports. In fact, it is a 12V 0.5A adapter - just sufficient for powering the ER-X itself, but nothing else. You need to provide at least 24V 0.5A (ideally more like 1A - 1.5A) on the DC barrel jack input or the PoE input on eth0 in order to use PoE.
I don't know what the power architecture of the ER-X looks like, but I suspect that if you attempt to power multiple "normal" 24V passive devices, it may cause problems with reliability (in fact, just using 24V 1A PoE input and eth4 PoE out to run older APs can sometimes cause unreliable operation).
All this to say that this mod is probably not a great option. A much better option is a PoE switch, or the ER-X-SFP (which has 24V passive output available on all 5 copper ports), or the ER-PoE-5.