HDMI-over-Ethernet extender only works through plain switches

Question

I recently purchased an HDMI-over-Ethernet extender pair like this: https://www.rubbermonkey.co.nz/Lenkeng-HDMI-CAT5E%606-Network-Extender-Kit Mine was off Aliexpress but looks the same. No manufacturer or model number was given, but I found the following manual, which allowed me to set the IP addresses to match the rest of my network: http://www.networktechinc.com/pdf/man181.pdf

This extender is sold as working over a standard IP connection, so theoretically can go through switches, etc. However I very quickly found that, while it worked with a direct Ethernet patch lead, I couldn't get an image when I plugged it through the switches I tried.

Long story short, I tried different Ethernet patch leads, HDMI cables, three different sources (two laptops and a security camera NVR) and three different destinations (two monitors and an HD projector), two different switches (both of the access point variety) and different resolutions ranging from 800x600 up to 1920x1080. I also removed other devices from the switches as a test.

In all cases the direct connection worked, but through the switch would not. Eventually I tried a couple of plain vanilla, unconfigurable switches. These passed the signal through without issues.

I have gone through the access-point switch configurations, and can't see anything unusual in relation to firewalls, switch management, etc. In all cases all devices on the network are configured to 192.168.1/24.

I also tried to inject the signal through my Ethernet-over-powerline network, and this also resulted in no image.

But there's a bit more to it than that, because the transmitter and receiver are definitely getting a connection: - If I connect through any of the access point switches, or the Ethernet-over-powerline network, there is no image - the screen is black. - However if I disconnect the Ethernet lead on the TX, I get "Searching TX". If I reconnect that and disconnect the HDMI lead on the TX, I get "Check Tx's input signal". If I reconnect them both, it goes back to a black screen.

For my desired connection I would like to use the two AP switches (which I am currently bypassing) and the Ethernet-over-powerline network. I have not fired up Wireshark yet, but thought I'd see if anybody has any other ideas.

Thanks,

Anton