Need help troubleshooting to find culprit on inconsistent wifi download speed across different devices

[mesangue]

Hi all,

 

I've got an CenturyLink Greenwave C4000XG modem & router device for a 940mb connection. For the first two months all was fine and dandy, but lately the wifi is grossly underperforming - but only on some devices! For example, my phone gets 340mb download speed on ookla's speed test but my wife's phone, tablet or the PS4 gets 20-40mb all against the same server.

 

1- No QoS configured that could mess things up.

2- Tried devices on 2.4GHZ and 5.0GHZ bands

3- Tried devices on Wifi within the same room

4- Tried disconnecting and reconnecting devices

5- Reset the modem 

 

Wired connection checks out fine (940mb download speed).

 

In all honesty, I have no idea what to do next. Cant find the root cause to fix...