Device lockup (Orange light) on home WiFi

I've been trying to troubleshoot a problem I'm having with a secondhand EX6100. I bought the device from a friend that had been using it without problems, but when I reset it and connect it to my network, the device locks up soon after and I have to power-cycle, even with no WiFi devices connected to it.

Initially I thought there were problems with the 5G, so I setup the device in 2G only, without luck. I've tried numerous settings, from leaving it basic config (wizard) to custom settings. I've reverted to stock firmware and then updated again. Nothing I seem to do makes a difference. I've lost count of the times I've reset this device.

The Ex6100 is connecting to a WNDR4500, both with latest firmware. I have zero problems with any other device connected to the WNDR4500, on either band.

I gave the EX6100 back to my friend to test again on his network and it worked fine for 2 days, where as I can only get a few hours at most, but usually around 5 minutes before lockup.

Thanks in advance for any suggestions.