[SOLVED] Android tablet DNS resolution error - router switch

[kunino]

I recently set up an old router as an ethernet switch (a BT Hub 5), and ever since have been having trouble connecting an android tablet to the internet.
I keep getting DNS resolution errors or "Your connection is not secure" errors with Chrome and Firefox, and other apps just refuse to connect at all.

All my other devices have no problems connecting, both wired and wireless. If I turn off the BT Hub then the tablet connects properly straight away, so the problem is obviously some sort of conflict created between the two, but I am at a loss as to what, exactly, the problem is, and how to fix it (short of ditching the Hub).
I have already turned off DCHP on the Hub, and have turned it off on the tablet, too. The static IP address is not shared by any other device. I have tried changing the DNS servers to the Google ones. Nothing works. Sometimes it does connect briefly, but it always reverts to DNS errors before long. So, any insight into how I can resolve this?

 

[bill001g]

Did you change the IP address of the bt hub so it does not conflict with your main router.

 

[kunino]

Yeah. Hub ends .2, main router is .1
And if it were a simple IP address conflict like that, I would have expected the setup to not work across the board, but like I said, it works absolutely fine for everything else. My PC and PS4 connected through ethernet cables, and my laptop and Nintendo Switch connected through wifi all work with no issues. It's just this one tablet.

I actually had a similar DNS resolution problem with my main PC when I first set up the second router as a switch.
The problem was fixed by turned off IPv6 in the PCs network settings. Unfortunately, you can't do that on Android devices.

 

[Alabalcho]

Make sure DHCP service on the "Switch router" is turned off.