> Unfortunately, only the yellow light (=> 802.3af) and not the green light (=> 802.3at) is lit on the switch,
I assume we are talking about the right-hand LED on port 5 - and not the left-hand LED which indicates the negotiated current speed on the ethernet connection.
> although this PoE injector should be 30W. Can someone explain to me why this is so?
802.11af/802.11at etc. performs a rather complex handshake when the link is powered up (plug in the cable, switch on, etc. ) - although there are also several legacy variants of this. This process regulates how much power is made available at the PSE (Power Sourcing Equipment) and how much power is available and can be drawn at the PD (Powered Device). Unfortunately, the fact that the switch is supposed to support both variants does not make it any easier.
> Is the injector supplying too little power? Is the switch defective?
Probably neither. As @Final has already pointed out, this is probably a compatibility problem, which typically leads to problems with more or less good injectors. Switches with built-in IEEE-compliant PoE/PoE+ and higher power levels usually work perfectly with the GS105PE.
The effective problem is that if the switch only displays an 802.11af power supply, only a single Class 1 or Class 2 (no Class 0!) PD can be operated.
https://kb.netgear.com/25541...
Since we don't know which firmware is installed in the box, there may be an additional hurdle to the recommended update. Possibly the bootloader (which contains a lot more that is relevant for PoE problems) has to be updated - but the latest firmware kits do not contain this bootloader. Maybe we'd better have a look at
https://community.netgear.com/t5... - there the paths are a bit easier.