@Mycroft I asked as I recall an issue with Dlink (I think) switches when communicating with TPLink devices, but obviously not here. Is the Synology a single NIC device? If not, does it perform the...

@crombinator there are logs in the admin area webpage on the deco. Browse to your main Deco's IP in a web browser (log in with your TPLink password) Go to Advanced -> System -> System Log

@Mycroft what switch do you have?

Try setting the limits to 1000Mbps manually and see if it changes anything.

If they are 'Unknown Device" or similar, it's likely an iPhone or Android device with network privacy switched on. Otherwise, you'll need to give a bit more detail about the devices connecting

I have a Samsung Tab from a similar era with almost identical behaviour. Try enabling the IoT network (or the Guest network) and restrict it to 2.4Ghz, then connect the Tab to that, making sure to...