So. Have a small networking story to share with some equipment at home. Just FYI in case you attempt something similar.

So my setup involves an ER-X and a PoE Unifi AP. I had the setup with passthrough PoE to the AP so that I'd use one power adapter for both devices. Everything was up and working fine, however I realized I had periodic slow speeds. No idea why. When the speeds dropped I would check all kinds of things and started blaming FLOW.

Ping to the ER-X would be fine but I'd have dropped packets to the modem. Direct connections to the modem would give full speeds but through my AP it would be greatly reduced and fluctuating. No idea why because the Ubiquiti should give me better quality connectivity (and control) overall.

After changing out some of the cables to CAT6 - this after I replugged cables and things worked from the slow speeds - the speeds dropped again and I couldn't figure why. I decided to try one last thing.

I used a power adapter for the ER-X and the PoE injector for the AP only instead of using the passthrough. So far it's been 2 days and no problems so far. Speed tests have been consistently good with or without other devices using internet.

If this works for the rest of the week with no other speed drops - it will be a lesson in overall power/usage by the devices. I had thought that the small power draw from the ER-X should not degrade the service from the AP - but it may not work like that at all.

So in summary - if you have PoE equipment that offers passthrough. Unless you're SURE that the provided power from the PoE will be able to supply both devices - use direct power. There's also the fact that the ER-X carries a fuse internally and limits the overall power going through - so the AP may need more for certain operations and it may be bottlenecked by being shared through the ER-X.

The solution would have been to use a PoE switch, but I didn't have one available and was just testing. But hopefully you won't make the same mistake or if you're considering something similar you'll know what to expect.