This has been a problem before but now that we upgraded from 50/10 Mbps to 120/25 Mbps I have to figure out whats the problem.

We have a DSL connection which is 100% stable and on a PC I get constant 120/25 throughout the day but wi-fi is super inconsistent both on 2,4Ghz and 5GHz event when the connected device is in the same room

https://www.youtube.com/watch?v=6ktGjcf2BWY

Here's a video of speed testing the wifi with my phone roughly 2 meters away from the router which is TP-Link Archer C6 (main router with PPPoE and VLAN ID) we have a TP-Link VR300 in bridge mode acting as an extender and shows the same behavior (Both on the newest firmware).

I tested this on my OnePlus 5, Galaxy A50 and LG G4 and other family members say the have similar problems with their devices.

It starts fast but then it deteriorates often to 0 Mbps and upload even lags completely. WEb pages often don't load and it's basically unusable.

I tried reseting the routers several times, changing wireless settings but nothing...