Okay, so to recap: You need 802.3af (or 802.3at) mode-A PoE injector/PSE, and a mode-A PoE-in switch, or a mode-A PoE splitter/PD plus compatible switch, across that two-pair section.
TP-Link says all its PoE switch PSE's are all mode A. However,
TL-RP108GE uses mode B, and only accepts power via PoE mode B. That is, it always uses pins 4,5 and 7,8 for power.
If we are to believe TP-Link community pages and several other online sources, then TP-Link
TL-PoE150S and
TL-POE10R pair will do IEEE 802.3af over mode A, i.e. using pins 1,2 and 3,6, only, for both data and power. Yet, the datasheet does not say for sure. (If someone has these, testing them with a cable with 4,5,7,8 disconnected would be excellent.)
On the downstream/PD side, The Pi Hut in the UK has some 802.3af mode-A compatible splitters:
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105026: 5V 2.4A USB-C for £9.50 -
Adafruit 3238: 12V 1A for £10.10 -
Adafruit 3239: 12V 1A, 9V 1A, or 5V 2A, for £25.40In addition to the splitter, you will need a switch that can be powered from the splitter output. The voltage is the key. I do know D-Link DGS-105GL and Zyxel ZyXEL GS-105S v2 (gigabit 5-port switches under 20€ here) take 5V barrel jack at less than 1A (2.5W nominal, so 0.5A).
You can find similar ones at e.g. Amazon.co.uk, like
DSLRKIT 802.3af PoE splitter, 5.5x2.1 barrel jack £9.50, (
same with USB £8), which claims to support both modes A and B. (DSLRKIT does make other PoE mode A stuff, too, but I haven't physically verified the devices, of course.)
You can get the above from anywhere else, of course, I just wanted to provide specific links so you can check.
Next, the supply/PSE side.
Unfortunately, the Tenda POE30G-AT is midspan/mode B only, and only provides power via pairs 4,5+ 7,8- so that won't work with your two-pair cable.
Devices that say they can supply IEEE 802.3af or at in mode A (1,2+ 3,6-):
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TrendNet TPE-115GIv2.1 injector
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TrendNet TPE-TG44g, 8-port switch
- DSLRKIT PSE4P1UGK, a Chinese gigabit switch with four downstream PoE ports
- If we believe TP-Link customer service, then e.g.
TL-SF1005LP, a five-port 10/100 802.3af/at PoE switch should work
Since GbE uses all four pairs, shouldn't all GbE switches providing IEEE 802.3af/at support mode A? There is even mode C, which uses all four pairs for both data and power. Well, UniFi ones use mode B only.
And finally, you can fix your current situation using not too many quid, if you don't mind hacking it together.
Pick up a fake PoE injector cable pair, like
this £3 pair off the Pi Hut, and two Abracon ARJMxx-104 MagJacks. Mouser sells the latter for around £4 apiece.
The idea is that we connect your Tenda POE30G-AT to the splitter's RJ45 with a standard Ethernet cable, all four pairs connected. The splitter RJ45 should have 24.9k 1% resistance between 4/5 and 7/8, so that the Tenda will supply it with power. Cut the wires around midway on the splitter, soldering the wires to the ARJMxx-104 MagJacks, power side to VC2+ and VC2-. VC1+ and VC1- won't be connected, because your wall cable has no wires there. You connect the MagJack to your wall socket with a short patch cable.
On the other side, another short patch cable from your wall socket to the second MagJack.
Here is a choice to be made. You can reuse the injector to make this side a nonstandard injector (by connecting the MagJack data and VC2+ and VC2- to the injectors RJ45, by basically cutting the injector cable and soldering the its RJ45 side to the Magjack). The PSE always sees the signature resistor, so this side would be supplying the 48V regardless of whether the connected device can handle that or not. If you clearly mark the cable for the TL-RP108GE only, then there should be no problem, though. Inside the wall, the two-pair cable will have 48V bias on its pair, so that needs to be marked so too. Whether the direct voltage is okay for the TL-RP108GE, would be easy to check if you have one end of an Ethernet cable, and a lab voltage source capable of 48V at 100mA. If you connect 48V to RJ45 pins 4,5 and 0V to RJ45 pins 7,8, the TL-RP108GE should just work. If not, it may blow up, but see my previous message, that still holds.
Or, splitters and devices that support mode A ought to handle being supplied the 48V, so any mode-A splitter could be connected to the wall socket, and that then to a switch (that takes 48V in).
Or, you can take the splitter's unused RJ45 (the plain one), and solder that to the chip side of the MagJack. The VC2+ and VC2- from the Magjack you solder to a male barrel jack cable. You connect the plain RJ45 and barrel jack (soldered to the MagJack) to a proper 802.3af injector's inputs, so that it does the downstream PoE capability checking. This active PoE 802.3af injector should be mode B, since that's what the TL-RP108GE wants, which should be common.
Or, you can take the splitter's unused RJ45 (the plain one), and solder that to the chip side of the MagJack. This is then a normal non-PoE RJ45. Use a DC-DC converter that can take 30-55V DC in, and drop it to 5V (since that's what most small switches want), and power a small switch using that.
All this said, I would personally just replace the damn two-pair cable with a shielded one, using the existing cable to pull the new one in its stead. Problem solved.